Electrical problem

[Tully]

I have a problem with my 71 GMC K 2500. All the lights work fine inside and outside until I turn the dash lights on. What happens is with the dash lights on the turn signal indicator lights and the high beam light come on dim and quit working properly. When I turn the light switch till the dash lights go off every thing works fine again. All I can think of in replace the light switch then if that doesn't work I would suspect the circut on the back of the benzel. I am hoping someone else has had this problem and can offer some insight.