2009 Toyota Highlander Trailer Wiring Harness installation instruction and wiring diagram can be found in the following file. DON’T pull on vehicle wires and/or wire harness to install the trailer wire harness. Pull only on the connector to uncouple electrical connectors. Nylon ties and adhesive foam strips can be used to secure the vehicle harness and the trailer wire harness. Before, make sure that all wires are properly insulated from ground. When installing the harness, make sure it is not perforated or cut by any sharp metal object, and many others.
For more details about the installation guide and wiring diagram, download the pdf document from the following source :