wiring gremlins

You may have multiple problems

FIRST thing to do is make SURE everything is off in the car, including trunk, courtesy, dome lights, and yank out the lighter. Get a charged battery, and hook up only the positive cable. Take your test lamp and wire it in series with the battery ground. If you have a drain or a short in the car, the lamp will light

Pull all the fuses one at a time. If that shows up nothing, unhook the alternator output stud.

So far as your dimmer, this sounds more like a cluster grounding problem, to me