I think a ground problem?

Ok here is the deal, I installed a Sony amp 1600 watt for my friend and 2 12 in sony subs. I used 8 gauge wire for power with fuse and round. The system worked for about 2 weeks then he took it out of the trunk, he disconected it. When he put it back it did not work.

I tested out everything and all the wires work. The only problem that I came across is that when the radio is off the ground wire for the system is negative, but when you turn on the radio, the amp light goes on for about a second and it goes off. Now when I tested the ground with the radio being on the ground gave off a positive. NOw what is going on? When I reconnecteed the ground to a different place in the trunk it worked for about a week but now he has the same problem. Is it the amp that is broken or should I reconnect the ground somewhere else and hope for the best?

next time show him how to take the fuse out from under the hood before pulling anything out. make sure he doesnt ground the remote coming from the HU cause it will usually fry.