[black98z71]

I need to change from a 4 pin flat connection to a 5 pin flat so that I can deactivate brakes on our new trailer with surge brakes. For OBS, I was wondering where I can tap into a stock reverse wire to wire into the 5th pin. I assume the wire would be in the bundle of wires underneath by the spare tire, but I was wondering which color exactly the one for reverse lights is.
thanks