[vettestr]

Steve,
I am not being a smart a$$ here but (If you hook up a wire from a 12 VDC source on one end then hook up other end to pin 7 on the truck side you will have what you are asking for).
Point I was trying to make was to keep from potential damage if ever used by ANY other person = truck = tow truck etc... and wired different than the Standard then a possible problem but adding voltage to a spare pin is probably a safe bet. I might suggest you hook up to pin # 4 that is now the charge or supply voltage. Again I stress the isolator so it will not drain tow truck when not in a charging state or condition.
So it will supply voltage when parked folks add series connected 6 VDC batteries in the trailer to run off of or add a second 12VDC tow truck battery with isolator between that battery and tow rig for starting reasons. The tow battery will not last a real long time without charge so that is why the series 6 VDC are so popular.