I had a similar problem with my '96 Safari van pulling a 26S. I spent about 6 hours testing all the pins and the only one not powered up was the hot wire for the brakes that I ran from the engine compartment through the firewall under the carpet to the rear and connected to the 7 pin plug. Turned out the wire was corroded at the area of the rocker panel but couldn't be detected visually. The local Trailer/Hitch dealer found it in 10 minutes and replaced a 12" section and I was good to go..So check for continuity if it's not a factory harness.
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs