I've heard you can use a specific Isolation Xformer from good ole' Radio Shack, but I would have no idea how to wire it.
I'd start hitting up goodwill or whatever for any kind of cheap stereo or receiver (small enough to fit inside the cab) with both a headphone jack and aux. input jacks. Tap into the speaker wires (from your pac) and wire that into aux. input. Get a y-cable if needed (to convert mono sound from the pac to both right and left channels on the stereo), and if desired, get a headphone extention cable (maybe with volume control in line). Cut the power plug from the stereo, strip the ends to expose wire and tap it into 120 inside the cab (or just plug it in another outlet, whatever floats your boat). I would think that's the easiest way, and would be easy to remove later if needed.