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Installing Trailer Wiring Harness in 2009 Mercury Mariner Without Factory Tow Package Wiring


My 2009 Mercury Mariner did not come with a hitch. However, it has the wiring adapter at the back of the vehicle. And, the power distribution box has the spaces for the three tow relays needed to power the wiring adapter. But, when I tried to install the relays, the fit loosely in the spaces and make no electrical connection. There is no friction what so ever. Am I missing a step?


Expert Reply:

The trailer wiring harness you reference, Draw-Tite # 118251, is intended for the Mercury Mariner that is equipped with the factory tow package (but will not fit vehicles with backup sensors built into the rear bumper).

For your 2009 Mercury Mariner without factory-installed tow package the correct wiring harness is Tow Ready # 118551. This part uses a powered converter that draws its power directly from the vehicle battery. It plugs into your Mariner tail light wiring and requires no relays or fuses be installed other than the included fuse for 12V power to the converter. I have linked an installation video for this harness on your same year and model of vehicle.

Since your Mariner does not have the factory tow package it is likely that the circuitry is incomplete and that the power distribution box does not have the electrical contacts installed in the relay receptacles. This would explain why the relays you tried to install fit loosely in the sockets.

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