Routing the Power Wire for a Trailer Wiring Harness Sep 17, 2018Some trailer wiring harness 4-Way and 7-Way connectors require a power wire to be routed from the harness to the vehicle battery. The power wire is required on trailer wiring harnesses for some vehicles because the electrical system on those vehicles cannot handle the amperage draw associated with trailer lights. Amazon Best Sellers: Best Towing Hitch Wiring ONLINE LED STORE 8ft 7-Way Trailer Plug Cord Wiring Harness [7-Pin Trailer Wire Cable] [Brake & Light Control] [10-14AWG] 7 Prong Trailer Light Cord Wiring Connector for RV 4.7 out of Tekonsha Prodigy P2 Trailer Brake Controller - 1 to 4 Otherwise if the truck has a factory installed 7-Way trailer connector that is a good indication that it has the tow package. That would mean you could then use wiring harness # 3040-P to plug in a Tekonsha brake controller such as # 90885. You can check for the factory connection point which would be located under the dash above the emergency..