I ordered a hitch from etrailer.com and a converter from Toyota but was unable to locate a wiring harness. I made a few failed attempts to order an OEM wiring harness and then decided to just make one myself. I took an existing 4 way wiring harness and cut off the vehicle side connector. I purchase a male and female 4 port power connectors from Radio Shack. The Highlander connector is located on the driver's side below the plastic storage bay. I then setup the connectors and wired them as follows.
Highlander wiring --- Trailer wiring --- Function Red --- Green --- Right Turn/Brake White --- Yellow --- Left Turn/Brake Yellow --- Brown --- Tail Lights White/Black --- White --- Ground
I hope this helps others...
Julian