I am finally getting around to installing a trailer hitch on my 2004 XLS and now am trying to remember what the "trailer towing/full size spare" option bought me. I can't remember if I ever saw the harness and where it plugs in. Anybody know?

 

Driver side left of spare tire well, are two tie down points. Lift cover if points to remove them.
Lift out spare tire well cover, then you can remove the cover that the two screws held in.
You will see the trailer harness connector.