Where to mount varies from rig to rig, For example on my Damon, since I do not have a driver's door, I took a length of 2x4, custom cut it, finished it, and bolted it to the wall under the driver's side window, (making sure I hit studs in the wall) I mount two CB's and a ham rig to that length of 2x4.
If the Coax is too short, you can extend it very easily.. You need what we call a Barrel connector (Double female) and a short (however long you need) Coxial jumper.
Make very sure the coax is connected to the double female, make sure the "Points" on the plug go into the "V"s on the connector, then tighten will with pliars, Do the same with the jumper, then tape over them in such a way to encourage the thing to remain well tightened. And on to the radio.
Again make sure the "points" go into the "V" notches and tighten well.
With the common PL-259 type plug (What is used for CB) this is the most common problem, not tightening the plug well so you loose the ground.