Expert Reply:
The ground wire on a trailer harness can be connected to almost any metal surface that is electrically grounded. Make sure what ever point you are wanting to use is free and clear of paint so there is direct metal to metal contact. The best place would be the frame. Check for other wires attached to the frame by a screw and connect to one of those.
The 2009 Toyota Matrix install video shows how and where to connect the ground at time 01:10. I will include a link to the video below.
expert reply by: Patrick B
