Electrical problems can even drive professional mechanics crazy. If I told you waht my lights do right now I'd get 15 different opinions as to what the problem is. Basically it's a process of elimination.

If I turn on the headlights all my lights work except the dash lights. Step on the brakes and the dash lights come on. Turn off the headlights and step on the brake and the dash lights come of as to all the exterior courteosy lights. All because the tail light housings are not completely tight. They are only finger tight as I have them loose for paint and body work.

Electrical ground problems are sometimes hard to diagnose as they produce mutliple symptoms.