How to wire a trailer plug - 7 pin (diagrams shown) - YouTube Click to view on Bing18:05How to wire a trailer plug - 7 pin (diagrams shown)Author: ecologicaltimeViews: 217K Factory Trailer Wiring? - GM-Trucks I would like to use it for back-up lights, which I installed on my trailer, but was wondering what it is usually for. 5) Extra Wire - in behind the hitch-mounted plug, coming from the harness, is another thin wire with an unique end on it, covered with a rubber protective "cap". Any idea what this wire is used for - assuming it's a GM factory Trailer Wiring Connector (7-pin) - GM-Trucks In 2014 GM decided to install their trailer wiring connector upside down on the Silverado/Sierra. The 7-pin connectors are designed so that after you plug in your trailer the flap comes back down on top of the plug and there is a small latch on the flap that hooks over a corresponding bump in the plug to prevent it from coming out.