I did something similar once. I gutted a full height PC tower except for the 12VDC power supply. I wired up a car stereo to the power supply, and then mounted a subwoofer and two mid range speakers in the side of the case.
It was a great stand alone party music station for bon fires and other outdoor events. I even wired a bypass so that you could run it directly off of a 12 volt car battery.