had the same thing happen to my 1998 t100 a while after i installed an aftermarket radio. here is the way i fixed it:
the factory radio is powered by the factory wiring harness that also powers the instrument panel and air conditioning control's lights. when you install your new radio it has its own wires for power (red, 12V), ground (black), and constant power (yellow, for the radios clock and memory). my new radio draws a lot more power than the factory radio did (85Wx4) and it caused me to send too much power through the rheostat switch (guage dimmer switch) about three times after putting new ones in and popping a circuit in the rheostat. i finally figured out that you have to disconnect (i just cut them and capped them) the power wire that powered the factory radio (i believe it was a blue wire, you can find out exactly which one in your haynes repair manual). in other words, the new radio drew to much power for the rheostat to handle and cause it to short circuit. because your new radio has its own power wire the factory radio power wire is no longer needed.
IN SUMMARY all you have to do is cut the wire that powered the factory radio (the blue one i believe) and make sure your new radio has its own power wire (red) and that should solve your problem, after you buy and install your new rheostat switch of course
hope this helps anybody who is having trouble cause it was a pain in my rear end to figure out!