Travel trailer lighting issue

[cwuwildcat]

I'm getting my trailer ready for the summer, and can't get the taillights to come on.  Brake lights and turn signals built into the taillights work fine.  I've tried them with all four lightswitch options on the Yukon.  I've even covered up the light sensor in the middle of the dash that controls some of the lighting activities, to fool it into thinking it's dark outside.

Here's my setup. 
An 07 GMC Yukon with 7 wire harness package built into rear bumper.  (Six blades in a circle with a female connection in the center of the six.)
92 Dutchman travel trailer, with wiring harness that matches up to the harness on the Yukon.

I've towed the trailer with other vehicles, lights all worked.  This is the first time I've connected it to the Yukon.  Bulbs are fine, I've checked them.  No blades are bent.  Any ideas?

[Humptulips]

Have you tried giving the plug in a good squirt of WD-40? Sometimes when mine sits a while some of the connections just won't work until I drive it a couple miles.
Corroded connections is a likely thing to look for IMHO.

[Todd_ID]

Is there a chance that the truck ground goes through the hitch?  Maybe pull it through the yard for 50 yards to see if it all starts working fine. 

[cwuwildcat]

It looks like it was corrosion.  Hit both connections with PB Blaster, let it soak for 30 minutes, and then "let there be lights!".  Thanks guys!