I agree with Roamer. I like my home made test lamp.. Take a standard 12 volt lamp socket with two leads, a License Plate Lamp socket from the local auto-parts store is a good choice, with matching bulb.
Put a clip on one lead, extend the other lead to a pin, (Safety pin works well here)
Now, clip the clip to first the negative battery terminal and touch the pin to the positive terminal.. Light. good, dark, bad battery.
now move the clip to the chassis (Any bare metal attached to the chassis) and again touch the positive terminal, LIght good, Dark, bad ground.
Now move out along the wire Roamer mentioned, You can touch a screw/terminal or pierce insulation with the point of the safety pin.. Light good, dark, you found your problem (it is between light and dark)
To attach the wire to the safety pin you can just push the pin through the wire, piercing the insulation twice, or you can strip off a bit and wedge it in the sprial spring part of the pin, even solder if you like.