dash lights on s-10 blazer

We had to replaced the thermostate and replace it with a manual one, now for some reason the dash lights are really dim when we turn the headlights on I don't think it is related but I was wondering if someone might know what I could start looking at?

I had the same dash light problem on my 84 S-10, also oil pressure dropped when I turned the lights on. It was a bad ground. Apparently the dash frame is hung on the body frame and tack welded at the lower corners. The tack welds had broken. After I put a couple self tapping screws at each corner, problem solved.