There is no control relay for that. The drivers are located internally in the TIPM. When the key is on, they send supervisory information, which may look like a voltage. Are the trailer bulbs partly lit??? If they are, you have a TIPM problem. If the bulbs operate normally, then the voltage you are seeing is the supervisory signal.
One thing about the advanced electronics in the newer vehicles, it is sometimes difficult to test/trace things, as there is so much more that just on/off voltage.