You should be able to get hte little black box with everything you need from any parts store, hardware store, ... The black box will take the seperate wires from the turn signal and the brake lights and combine them into one signal for your trailer without affecting the truck works.
Just be careful where you splice into the trucks wiring. Try and find some waterproof splice connectors or silicone the snot out of the connection. No matter how well you tape over the connection, water will get in there. A coupe of years down the road, and your brake light wires will corrode through. Then you will need to go back to the hardware store and get a wire stretcher to splice that section out of the wiring harness. PITA!