There is more than one way to skin the rat as it were. Here are some diagrams for different signal stats. From a Jeep site but the same principles apply:
http://cj3b.info/Tech/SignalWiring.html
At the bottom of the page is a link to the "Tech Tip" from this site. And to clarify, the power from the brake switch comes after the switch. I know sounds funny, the switch has to function to tell the signal stat the brake pedal is depressed. So power to the brake switch. Then from the other side power for the brake input on the signal stat. The old brake light wire that used to go from the switch to the brake lights is not hooked to the brake lights any more. It goes to the signal stat and, then goes to the tail lights. The problem is the existing brake light wires running under the truck. Those are the wires I suggested you disconnect for the testing. I don't think the signal stat is bed yet. There is more to troubleshoot before you get to that point. Follow the wires carefully. Use an Ohm meter to confirm all connections are right. Good luck and let us know how you do.

Steve H