12 volt exterior clearance lights showing negative power, not positive

All the 5th wheel tail and clearance lights were working fine when....i wanted to wire a backup camera for my 1996 Glendale Golden Falcon using the reverse/backup light for the power source.  Trouble is it didn't have a backup light in the tail light units, however the trailer harness had the wire dead-ending at the trailer's 5th wheel hitch plug.  I ran a single wire to the rear and got a deal on a set of used LED tail lights that had backup lights/tail/and signals.  All LED's worked fine when touched on a battery.  I also wanted to connect a small side signal light at the rear side quarter.  All lights formerly were incandescant bulbs.  Running a few new wires for the rear/side signals, using power from the tail light signals was fine, but when I powered up I had signal and backup, but no tail lights.  The LED's used a separate positive and negative wires.  Checking tail light polarity with an ohm meter found 12 volts with a negative sign in front, signal and backup lights showed regular 12 volts.  Connecting the harness to the truck blew the tail light fuse.  My Chevy has separate fuses for the trailer lights.  No camera has been hooked yet, still trying to chase the wiring problem.  More over, the ohm meter indicates a short at the wiring plug intermittently, sometimes working, sometimes OK.  I've only fiddled with the wiring by pulling the tail lights feeding a foot or so of wire, nothing else.  Can LED lights be connected into the line with the old incandescent fixtures?  Tail light power good, disconnect, then shorts to negative. Wow, I'm puzzled!! 

Is there any chance you have any of your LED lights wired backwards? That is to say the positive terminal is grounded and the ground is going to 12VDC.  LED's are actually diodes, this means they are designed to conduct electricity in one direction only. Also, can you explain a little more about the negative sign condition on your voltmeter? If your black (negative) lead is connected to chassis ground and your red (positive) is connected to a 12 VDC source there should not be a negative sign. Sounds like you have actually identified, if not yet located, the problem when you say there is a short at the wiring plug. I would carefully check every wire there until you find the short to ground, remembering that an LED can cause a short to ground in one direction of DC voltage, but not the other direction!

Thx for the reply HS.  The negative sign on the voltmeter would occur intentionally by reversing the test leads - ground onto the positive side of the battery and red onto the negative post.  The meter would show 12 volts with a minus sign in front.  Well, I get that reading when testing the LED tail light wire and it doesn't light.  As you say LED connections are pos/neg whereas, for example the old clearance light wires are not marked and light up either way.  I'm not changing any old clearance lights and have only connected two new LED 'clearance lights' on the rear side to act as rear side signal lights and one tail light at this time.  I'm pulling the other 'old' tail light assembly today where I connected the new LED side light to check wiring.  What's frustrating is all ground wires and tail light wires are bundled at the large cut out for the tail light I'm working on so it would be difficult to mess this up, but I'm running out of hair to pull out!  I've drilled some 'chase holes' and will work this today.  Chase holes will be covered by new reflectors!

Thanks for clarifying the negative voltage. I wanted to make sure we were on the same page. In my world, audio electronics, there is actually negative voltage, but I've never seen it in automotive applications. Sounds to me like it's still all about that short. Right now it doesn't matter if the short is 'the' problem or just part of the problem, it's the first thing that should be remedied. I understand your frustration. I only wear a hat when I'm working on frustrating projects. It's how I have kept a (reasonably) full head of hair.
Let me know if there's any way I can help. Sometimes it just helps to keep rambling through all the steps and something eventually jumps out as that 'Ah ha' moment.

The only way you can get a negative reading is if the point you're using as ground is being pulled positive.

An LED assembly installed backwards, so it's shorting the ground to 12 volts when it's turned on, would do this.  Whether it shows as a short at the connector depends on the integrity of the ground.

For example, a reversed LED fixture in a taillight or marker light (they share the same hot lead) will short out and pull a bad ground positive when the tail lights are on.  If you measure between that ground and the 0 volts on a turned off stop light or turn signal wire, you'll see -12 volts.  The shorted LED on the tail light wire would only show as short at the trailer connector if it's connected to a good ground.

Are you sure the LEDs have + and - wires, or could they be two + wires to select dim (tail light) or bright (stop light/turn signal) levels, with ground connected to the shell?  If so, they'd still light if the two wires were placed across a battery as long as the ground was left floating.

Better LED light replacement units contain what is called a Full Wave Bridge Rectifier. 

The idea is this.

Imagine a plumging system you want wate rto flow ONE WAY and one way only no matter which way you hook it up.

You use four check valves.. Hook them up right (the pipe you want water to flow one way in has two "Outlet" ends hooked to ione end and two INLET ends to the other) The remaining ends are hooked one outlet and one inlet to the two hose connections)

Now no matter which end of the assembly you pressurise, water flows only one way thorogh that one pipe.

That's how a bridge rectifier works.. Same way exactly.

If you plug such a light in with the NEGATIVE on the "Tip" (Assumes bayonet base) it works

If you have POSITIVE on the tip (normal) it still works.

If you feed AC to it.. Yup, it works.

Thanks guys, that's the puzzle Lou, all the ground wires (white) for the rear lights are crimped together with one crimp, similar to an AC marette, but a crimp style.  The LED has separate positive wires for the tail light and the brake/turn signal, but a common ground.  The reverse light beside this has a separate ground (white) and a positive.  All lights work properly when touched to a battery and won't work when these leads are reversed.  So I'm figuring there's no problem with the fixture.  Now thinking the harness plug itself may be the problem - it's moulded into the wire, I've cut it off, used the ohm meter to write down the correct corresponding colours and am applying power from a 12V battery WITH a 15 amp inline fuse.  THIS FUSE HAS NEVER BLOWN!!!!!  Even though the volt meter shows negative power at the tail light wire and applying power to the brake/turn wires  and the reverse light, still using the same ground wire in the bundle.  Yet when the battery is disconnected and an continuity meter is applied, a short shows, well, at least it beeps until I start detatching wires at the back.  These wires can change the 'beeping' as well, depending on their configuration, hence my now drilling some 'chase holes' so I can see which wires run to the side lights, etc.

Now, during the first side signal I hooked up, one side worked, the new side didn't and the electric brake controller showed something wrong. When I did the full connection to the truck/trailer plug, all was fun, in other words, I think connecting the electric brake contoller to the truck may have grounded the brakes and completed an electronic connection which the signal lights liked.  Does this make any sense as in my reading, the brakes don't care which is a positive or negative connection.  I'm trying this next after I check wiring connections for the umpteanth time.

Was the trailer hitched to the car during your troubleshooting or just plugged in via the cable?

A high resistance or open ground through the 7 pin connector coupled with an intermittant or missing ground through the hitch would explain a lot of the problems.

An then came the 'AH HA' moment.  No, the 5th wheel was not hitched to the truck, but that didn't contribute to the problem as I could tell.  I imported an analytical mind, a retired programer as I recall, part time auto hobbiest and a good friend who figured out I was reading the printed diagram wrong for the new truck's plug I wired into the side panel.  Up to that point  I was using the bumper plug which worked the trailer lights fine, once I had the lights wired up.  He noticed the diagram on the bumper plug's cover and saw I had the ground and the running lights connected opposite on the new side plug.  How the other lights worked I'll leave for another day to figure out as today, day three, was an eight hour job, but things are working fine.  And I'll figure out the printed diagram another day as well.  Thanks again for the posts and help everyone.

Congratulations! Victory is yours. You got to love those "Ah ha" moments!"

Lou was correct in that the point I used for the ground in my truck's new side plug was being pulled positive as this was the point I should have used for the tail lights.  Conversely, the point I used for the tail lights was the grounding terminal, that is current going to it when I turned on the running/tail lights.  I completed the 'short' when I turned on the manual running light switch and sent current to the rest of the grounded system, knocking out the running light fuse.  I had no problems with the bumper plug as this was wired correctly.  Now that I made my way through the alligators, the swamp has been drained and my 5th wheel now has a back-up camera activated by the trailer's newly installed LED backup lights/running lights/brake/signal light panels.