My guess, is what already assume happened (by mentioning the amp)... The lighter in my CRX is only on with the car, so I assume yours is the same... Some ppl, specially those who install stereos themselves, dont know how to wire the amp, to where it only comes on with the car... Some decks have a remote turn on lead, most dont... So a viable option, is simply wire up to a fuse, or other wire, thats only on when the car is on... So what I think happened, is the guy either spliced into the cig lighter wire and never hooked it back up to the lighter, or he hooked it up, and it came loose... Trace the wires back, chances are he spliced it right near where the radio goes somewhere (since they are in the same general area usually)...

If u cant find the problem at all, I say just wire it straight to your radio positive (either on all the time, if u want it on all the time, or with the ignition on if u only want it on with the car), and then ground it out to the chasis somewhere... I reccomend you dont just ground it out to the wire on your radio!