Do you have the load leveling system? If I understand it correctly, there is a OEM harness that plugs into a dead-end plug near the air compressor on these vehicles that comes out to a 4 pin flat plug. Otherwise, you'll probably just need to get one of the "T-type" connectors that don't require any splicing. I believe places like Hoppy have them
http://www.trailerwiring.com
Good luck...Glen