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