I have an ARE cap on the back of my t, the wires are rundown to the back left of the truck and patched into the back left taillight brake wire. The light on the back of the cab is turned off, (it is quite possible that the bulb simply was removed)

You might be able to call the cap manufacturer and ask them too.. The extra weight on the back makes the truck handle way nicer, especially when it is wet/ snowy out.