Re your rear speakers...in my experience, the magnets are prone to falling off the speakers (no doubt aided by the volume and bass that my kids crank up). With the stock sound system driving the speakers, both the magnets were dropped off the rear spekers, thus making them go dead. They were not a standard size, and required some adaptation to get 5 x 7 speakers into the stock Volvo speaker housings. Good luck tracking down the problem.
Doug