GMC 2003 Yukon Owner's Manual page 356

The seven-wire harness contains the following trailer
circuits:
•
Yellow: Left Stop/Turn Signal
•
Dark Green: Right Stop/Turn Signal
•
Brown: Taillamps
•
White: Ground
•
Light Green: Back-up Lamps
•
Red: Battery Feed
•
Dark Blue: Trailer Brake
If your trailer is equipped with electronic brakes, you
can get a jumper harness (electric trailer brake control)
with a trailer battery feed fuse from your dealer. This
harness and fuse should be installed by your dealer or a
qualified service center.
If you need to tow a light-duty trailer with a standard
four-way round pin connector, you can also get an
adapter.
4-74
Instrument Panel Jumper Wiring Harness
This harness is included with your vehicle as part of the
heavy-duty trailer wiring package.
This harness is for an electric brake controller and
includes a trailer battery feed fuse. It should be installed
by your dealer or a qualified service center.
This harness is new for your vehicle, and you will not be
able to use a harness from an earlier model year.