The wires at the back up switch are a constant hot and a switched hot. You can confirm this by studying the SOBill diagram posted early in this thread
The red trigger wires do not carry a ground, they carry a +12 V.
It is best to tap into the back up light wiring in the tail light bucket area to make sure you are getting the switched tail light power, not the constant 12 V hot that goes to the switch.
Larry