sorry to jump onto this thread me being new and all but the problem sounds similar to my own im currently experiencing.
my instrument cluster will light up the trip meter and the other warning lights such as dipped beams, fog lights, oil light battery light etc but the main back light for the speedo and rev counter etc will not light up...replaced the fuse initially but when turned the ignition and tried to turn the lights on it wouldnt light up the back light, thinking it might be a blow bulb or something took the cluster out and checked all the bulbs and they checked fine put it back in checked the fuse again and ofcourse it had blown... went through 3 fuses hehe they keep blowing...
the reason i says sounds similar is because when i got the car... second hand.. there was no radio in it so i fitted my own which of course dims and brightens when you turn the lights on... is this going to be the same thing do you think? a rogue frayed orange wire? if i take out the radio unit will it be easy to find and sort out?
its a 318i btw 1996.