If you do a quick search you might find better directions, but there are a few locations. The easiest location for me to get to was behind the passenger side kickpanel. Remove the pass. kickpanel and you will see wires going into either blue or white plugs. The reverse wire is the light blue wire located of the lowest white plug. To double check you got the right one, you can hook a voltage meter up to it and when the truck is in reverse you will get around 12 volts (give or take) and when you take the truck out of reverse, the meter goes to zero. If this doesn't help, I would do a search so you can see pictures.
I put an aftermarket on. I will have to try the way Trefider said, I heard that somewhere else. Could someone else help confirm the reverse wire in passenger kick panel. Thanks for the quick replies guys. Got to love TS!