Flat plug wiring question...??

[tootall]

The boat trailer has trailer brakes but, they are not working, so im not worried about that but, The trailer has a five pin flat plug.. My 05 Ford pickup has a standard 4 pin flat plug.. When I hook it up the male plug on the trailer has one pin still exposed... due to the truck only having a 4 pin plug. Marker lights running lights and turn signals work perfect until I hit the brakes and then my running lights go out.. when I let off the brake petal they come back on... Can anyone diagnose this problem for me?? Thanks!

[CatFan]

Your ground isn't connected well to the tongue of the trailer.

[jus10keith]

The extra pin on the 5-Flat plug is for the reverse lock out on the surge brakes. This pin is also for the back-up lights on the trailer (if so equipped).