i am working on adding a single pole double throw switch in line with my trailer back up light that is powered by my tow vehicle, I will wire the switch so that when I switch the back up light on it will operate from trailer battery power, and be disconnected from the pow vehicle power until I turn it off, and then it will automatically come on when the tow vehicle is in reverse
hot power for the light would go to post 1P , the tow vehicle back up hot would be connected to 1T , and the trailer battery hot would be hooked to 2T
-sorry I didn't understand the 4 pin plug part-