a few thoughts...
I believe tech1234 is correct, they are rear window defrost wiring... they would have 12V IF the van had the rear defrost option (and the defrost was turned on). If you have the option, the computer in the van would have been programmed to provide power... no option = no power.
I would suggest you NOT use them as speaker wires...It's easy to connect real, dedicated speakers wires from the upfit connector to the passenger back corner column. (probably a few minutes to snake up the B column and across the wiring tray above the door/window area)