[Degtr]

Does this happen when you put any other load on the electrical system (e.g., turning on blower, rear defroster, power windows)? If not, I would first look for a short in the speaker system, assuming that everything was fine until now. If you just put the system in and it has been happening since you put it in, it may be pulling more juice than the stock alternator and electrical system can provide. You may want to look at how many amps it's pulling.