It couldn't be the Amp. They are powered when the ignition comes on. I just swapped in the Mach 460 from a 01-04 in my 93. So i know the wiring like the back of my hand. I have a few extra amps here if you want to try one.
part No's f7zf-18c808-aa
All the mach amps from 97-04 are the same btw, the older ones will work too, but I don't have a pat number from those..
To test it, turn the ignition off, find the fuse that powers the amps, pull it, grab a test light, touch the clip to one side of the fuse panels fuse contact, and the probe to the other one. If the test light lights (even a little) you have a parasitic current draw somewhere in that circuit. If the light dosnt light, go through the rest of your fuses in the same manner, and find the one that lights your light, that'll point you to the circuit with the problem.
Oh yeah, if it is definatley the amp, lemme know, I've got mine outta my '99 Cobra in my attic. I'll crawl up there and find ya the one you need.