Troubleshooting Stray Voltage in 2009 Toyota Highlander Factory Trailer Wiring Harness

Updated 07/16/2018 | Published 07/14/2018

Question:

I have a 2009 Toyota Highlander. The Highlander has a factory towing package with a connector near the left rear tail lights where a straight wiring harness plugs in. This has a factory plug on one end and a 4 wire trailer plug on the other. No black boxes, other connections or wires between the factory plug in the Highlander and the trailer plug on the other end. Recently the LED trailer lights leaked and died. I replaced the lights but then noticed that the running lights stayed on, even with the car lights switched off and the ignition off. After verifying all the lights front and back on the Highlander were off I unplugged the trailer and checked the connector in the Highlander coming from the factory wiring harness. The brown wire running lights has 12V always. This isnt just a phantom voltage as I can plug the trailer in and there is enough current to light all the trailer running lights. Next I pulled the fuse that protects the trailer light circuit. With a voltmeter attached the voltage dropped very slowly, over the course of several minutes like draining the voltage from a capacitor. I replaced the fuse and rechecked all the wires in the plug and now see 12V always on the brown and the yellow wire. However, the voltage is slowly dropping on the yellow wire like it did when I pulled the fuse. Any ideas? Is there some kind of solid state relay or black box that controls the trailer lights in the Highlander? If so, where is it?

asked by: Skip M

Expert Reply:

Your Toyota Highlander's factory wiring should include a converter found under the dash of your vehicle. It sounds like your trailer may have caused an issue with your converter. If this is the case, I recommend using the T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector # 118449 which will give you the 4-way wiring harness, but also the converter for under your dash.

expert reply by: Jason S