How to wire a breakaway switch, which wire is which?

Bring out the multimeter and see if you have voltage to one of those wires. They may both be for the brake leads and you may have to supply a power lead. If you have power to one of the wires and no power to the other , connect them, as posted, to the break-away switch and check to see if your brake magnets are powered by pulling out the pin. You can do that with a stud finder or compass at each drum. Have you checked your trailer's brakes using the truck's harness?