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?
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.
Wire one switch to each speaker using the positive wires.
there you go.