I just bought a used 2008 Tahoe boat. The trailer has a 5 pine flat plug. I've never seen that before. The guy I bought it from wasn't sure why it had 5 pins. Any ideas? The trailer doesn't seem to have electric brakes.

 

It ties into your reverse lights, and disengages the trailer so it can be backed up. Without it, you will not be able to move the trailer in reverse.

 

The above poster is correct. The fifth wire ties into the backup lights. On a trailer with hydraulic surge breaks, backing up compresses the hitch coupler and activates the brakes, especially if backing up hill. That fifth wire activates a solenoid valve in the trailer hydraulic line blocking pressure to the breaks when the reverse lights come on allowing easy trailer backing without fighting the brakes.

 

yepers, backup lights on the trailer.