2004 GMC Yukon Owner's Manual - Page 376

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. 4-78