i think i would take the vehicle to a local stereo shop, and have them take a listen. imo though, it is probobly an amp.
http://www.carstereohelp.com/cadillac.htm check with these guys...they are the best over the phone advise for bose. and you dont even have to take the phone to the car.... lol. serious though, they will help, give them a call
rickyd