the lights and dash lights are not illuminating

i have a 2002 pontiac grand prix and my heat, headlights, interior lights, dash screen, and rear desfroster dont work when its cold outside and work when its warm...i have to keep my switch on 5 for it to even work if i move it from 5 to 4 or less everythi changed my blower motor resistor because it only works on 5 (sometimes) and that didnt help...last year i was messing with wires under the dash and i thought i found the problem because it was fixed for the rest of the winter and then it was summer and everything was fine...now its winter and it only works when it wants to...i tried to find that same wire that i found last year and i cant find it now...ive messed will all the wires and its not doing anything...ive and 3 shops look at it as well as myself and nobody can find the problem