To install the 2008 Toyota Highlander Trailer Wire Harness, do the following: DO NOT pull on vehicle wires and/or wire harness. To uncouple electrical connectors, pull only on the connector. Use nylon ties and adhesive foam strips to secure the vehicle harness and the trailer wire harness. Ensure that all wires are properly insulated from ground. When installing the trailer wire harness, make sure it is not cut or perforated by any sharp metal objects. Etc.
Get more info about 2008 Toyota Highlander Trailer Wire Harness here (16 pages of PDF file, source: sumbiz03.home.comcast.net).