dash lights out

i have a 1987 Firebird and the dash lights went out, just the ones behind the speedometer and the fuel gage. The check engine, oil, and brake lights all still work. i have an after market gage hooked up to the dimmer switch as well and it just turns on now and wont dim and the interior light only works when you open the passenger door now. Is this all connected to the same problem? I checked all the fuses twice, so i know thats not it. And advanced auto told me to check the bulbs for the dash lights, but if they burned out that shouldn't cause everything else to not work. Any ideas?