When I installed the ReadyBrute Elite tow bar between my (previous) Class C and towed car, I bought their indicator light. It had me run from a hot wire under the dash to the light, then a LONG wire to the back of the RV where there was a connector. The other side of the connector went to GROUND. The tow bar had a matching connector that just grounded the two wires together when the tow bar activated the brakes. I was unhappy with that because I wasn't interested in when the tow bar brakes activated, I wanted to know when the car brakes activated. A subtle point, but an important one to me. I put a 12v normally open relay between the towed car's third brake light hot wire and ground. Then I ran a long wire to the front of the car and installed a matching connector, and made a 4-ish foot long jumper to go from the front of the car to the back of the RV. After that, if the brakes on the car DIDN'T activate, I knew it (because the light did not come on). Or, if the brakes got locked on and dragging, I knew it (because the light stayed on).
Once installed, it was absolutely no problem. Connect the jumper wire at both ends and it was done.