Quote Originally Posted by Shoebox View Post
I have the 7 pin connector, but it doesn't lock out the brakes. Had to use the lockout key that came with the boat.
Sounds like you have the round 7 cavity connector. In this case, only 5 pins are used for the trailer harness. Ive not come across a round 7 with only 4 pins active. So id say your trailer has the electric lockout solenoid.

I would google the pin-out and check the proper revers pin for voltage, with the truck in reverse.