Bought new set of wiring with trailer module and right plug for dad in law's car today, fitted it on perfect! But...

Mid wiring, theres earth white wire which is connected to earth point no problem, only one i couldnt find is yellow plug that seem to fit a reverse thing, where or what should i plug into that?? My car wiring had no such wiring sticking out like that! Different brand! Mines towtrust, his is witter. Cost £48 for signal electrics set up pack