Well, most likely since it went out when they installed the cd player, I would assume that they either cut a wire that they weren't supposed to or pulled one out of place. A 12V plug adapter is very simple. Just one ground and a hot 12v wire. You can try and trace the wires back to find the mistake which will probably not be easy or fun, or you can just find another hot 12V circuit and a new ground and just wire it back up. In my experience, it is usually much more efficient to just set new wiring instead of trying to troubleshoot bad wiring.