Towing Issues: Trailer Wiring Problem, ford ranger xlt, 4pin connector

Question
I have a 2000 Ford Ranger xlt supercab 3.0 FFV. I bought the truck used and it was wired with a 4pin connector. I checked the connections with an amp meter and all are live (including the brake lights). I bought a brand new trailer kit, put it together, and hooked it up to the Ranger.
My issue: all the lights work on the trailer (turn signals, running lights) but when I press the brake pedal all of the trailer lights go out. Any ideas here?  

Answer
Two ideas, a bad ground or some wires are bare and touching each other.  

Run a separate ground wire from the chassis of the tow vehicle to the chassis on the trailer to eliminate a bad ground or missing ground.  

Then start tracing wires, especially anywhere they have been cut or spliced.  Re-do all connections while checking that they are hooked up right as you go.

Don