GMC 2003 Yukon Owner's Manual page 357

Four-Wire Harness Adapter
This adapter is 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.
Connect the adapter with the tab pointing up (see
arrow). The flip cap on the vehicle's seven-wire harness
will lock onto the tab and help hold the adapter in
place. Plug the four-way round pin connector onto the
adapter.
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