The plug in should be there already. In my machine it is up to the left of center (looking from the rear) and you will find two ends of wiring coupled together. Unscrew the plastic ring and you will find the plug-in sockets to connect to the trailer wiring. I was lucky to get a hitch bracket used that carried the trailer plug-in and the wires that connect to the plug-ins on the truck. If you don't have one of those look for a used one because over here the plug and wire assembly is very, very expensive.
-Dai