Towing Issues: Trailer Wiring Problem, ford ranger xlt, electrical wiring system

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
while they are about a half a dozen things that can be wrong there, I am wondering since you said that all the wires are live.. If they are all hot all the time even if the truck is turned off that is bad...

Now if they all go out when you hit the brakes, then there might be a ground issue in the wiring.. That would be my guess as the best chance...

Have it checked out by a trailer shop and see what is wrong. I know that there are direct plug in connectors which are made for each vehicle... And as with any electrical wiring system, just the smallest bare wire can cause major headaches in finding it or correcting it.