On most trailers the electronic lock-out is tied to the reverse lights on the vehicle. The idea is that if you were on a hill and the rig started to roll backwards, the trailer brakes would stop it...when the reverse lights are on the solenoid releases the trailer brakes as it "knows" you mean to go backwards. One of the four hot "pins" on the connection needs to be connected to the reverse lights on the SUV. Check the fuses/relays for reverse lights.

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