Start with the basics. If the controller is not "seeing" the trailer:

  1. Split the circuit in half. Make sure there is a controller output signal at the connector on the tow vehicle.
  2. Going to the trailer, and assuming you have output at the connector, make sure that those signals are actually getting to the devices (brakes) they control. You can also do this with a simple continuity check from the trailer side of the connector to the brakes.
  3. Check your grounds. Most of the time the problem is there. Check from the brakes to the trailer frame, and then make sure you have a solid ground connection back to the tow vehicle. In many wiring senarios trailers usually ground through the hitch, and this is not always a good solution. I often like to run a seperate ground from the trailer, through a connector, to the tow vehicle.
Hope this helps.