Why not wire up a sound system using indoor outdoor marine speakers in the bed of your truck with a switch in the cab to turn them off and on?
http://www.12volt-travel.com/65marin...0w-p-4743.html
These speakers, then cut a hole in your bed or mount them in a place that wont get in the way, run cables from your existing speakers taking notice of the negative and positive leads. Put a switch in there so when you drive down the road you don't blast people your music.
http://www.12volt-travel.com/toggle-...54_75_631.html
Wire one switch to each speaker using the positive wires.
there you go.