5-way plug boatmate trailer wiring
 

I'm trying to figure out my wiring on our new boat's trailer (boatmate). I have everything down except for the blue wires. I traced 2 to the clear LED/reverse lights & 1 to the trailer brakes. But when I hook up the trailer brakes the actuator doesn't engage. Did I miss something somewhere?

Going from a 5-way to a 7-way so I skipped the positive plug on the 7-way. The rest were all used (ground, left blinker, right blinker, stop/markers, reverse, and electric brakes).

The brakes are engaged by the coupler. 1 of the wires for reverse disengages the brakes so you can back teh trailer up a hill. That may not have answered the question, but hope it helped.

yup. Answered it perfectly I think. I wouldn't notice anything yet since I wasn't backing uphill, correct?

If your surge brakes are working right but you don't have the blue wire connected you'd have a really hard time backing up, even on flat ground.

The actuator engages automatically when the tow vehicle is moving slower than the trailer. No electrical interface. As the truck stops the trailer pushes against your hitch compressing a small master cylinder inside the tongue. The blue wire activates a solenoid that redirects the pressure generated by the master cylinder in the actuator back into the fluid reservoir when the vehicle's reverse lights are on to prevent the brakes from resting your efforts to backup. When the solenoid is disengaged/powered off the pressure goes to the brake calipers.

Checkout some of the animations here:
http://www.ufpnet.com/Actuators/tabid/54/Default.aspx