It indeed should have been a fairly simple hookup...not a huge amount can go wrong on 4 pin trailer connector. Your's obviously has a problem and I think you've kind of already identified the cause...the connector does not have a functioning circuit that goes to a ground on the truck.
On a 4 pin connector, one wire is tail/running, one is left turn/brake, one is right turn/brake, and the 4th (the one that's different in the plug) is the
ground. A really simple way to test the various circuits without getting involved with wiring colors (which sometimes aren't the standard colors) is
1. Go get a cheap ($1.95) test lamp at your local auto parts store.
2. Then attach the test lamp's alligator clip to a good ground on the truck (like a bumper bolt that's been cleaned up enough to get some shiny metal showing).
3. Then turn on the truck's parking lights as well as the 4way flashers...this should provide some kind of voltage at every terminal on the 4 pin connector except the ground pin.
4. Finally, stick the test lamp's probes into each of the non-ground pins on the truck's 4 pin connector...you should either get a steady light (indicating you stuck the probe into the tail/running light wire) or a flashing light (indicating you stuck the probe into one of the two turn'/brake lights.
If only 1 or 2 of these wires light up the test lamp, you'll have to do some troubleshooting....starting with the fuses in the fuse box under the hood.
If the connector passes the above tests, then all you have to do is find and fix the reason its ground doesn't actually go to ground.
