Thats right Chris... I just bought a new boat a trailer and it has a 5 pin and surge brakes. My dealer set me up with the 7 to 5 pin i needed and i was away.The fifth wire is used when the trailer has disk surge brakes. Unlike drum brakes on a trailer it is impossible to back up the trailer without disabling the brakes, something will brake or you will drage the trailer tires and grind down the tires. A fith wire is needed to open up a solenoid that causes the surge brakes not to work when backing up the trailer. The fifth wire is hooked up to the back up lights to activate the surge brake solenoid.
There is available through various vendors a 7 pin round to 5 flat adapter. Also available are two 7 pin round to 5 flat pig tails that stretch to either 3 or 5-6 feet.