I would back track the stereo.
The aftermarket stereo probably has a short in it some where
Since I am not sure how it is connected, I cant really give you a "culprit"
(ie. if it is a complete rewire, if they used a plug adapter for the stereo connect, and all that is added into all that. even with that info, there are still alot of wires that can be guilty)
You could get a wire tester and go through the grueling pain of testing each wire on the back side of the head unit.
