GMC Yukon Denali 2003 Owner's Manual page 297

Instrument Panel Jumper Wiring Harness
This harness may be 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.
Four-Wire Harness Adapter
This adapter may be included with your vehicle as part
of the heavy-duty trailer wiring package.
Use this adapter to connect a standard four-way round
pin connector to the seven-wire harness on your
vehicle.
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