blowing a fuse, question Options

[Batman722]

this is the situation.
A co-worker has a 94 GT coupe and he has an aftermarket stereo (the stereo was put in professionally a while ago with no problems). Yesterday the 15amp fuse for the cig/radio (interior) blew. He replaced and it blew right away again. Tonight I took off his stereo surround and cheched things out (looked for any shorts) and found nothing. Everything looks in order. I check the lighter assembly for any bare wires or tears in the sheaths and the same for the back of the stereo. I unplugged the lighter assembly just to be sure and still had the same problem. I checked the fuses in the bay and found that a few of them (radio and dome) had the wrong amp fuses in there (he had ones that were higher than they should of been, ie 25 in the 15). I replaced the higher fuses with the correct ones and still the same prob. The interior cig/radio fuse needs a 15 but holds (doesn't blow) with a 25.

I know he must have a short somewhere, but I didn't find it.
I know that having a 25 in there is only correcting the problem temporarily.

the question
where else should I look ? has this happen to anyone else ? should I check each wire (at least the acc and the constant) from the stereo next ?

any suggestions/recommendations ?
thanks guys