GMC 2005 Sierra Denali Owner's Manual page 272

Four-Wire Harness Adapter
Use this adapter to connect a standard four-way round
pin connector to the seven-wire harness on your
vehicle.
4-64
This adapter is included
with your vehicle as part of
the heavy-duty trailer
wiring package.
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.