The guy with the trailer brakes needs to look at his trailer brake control unit. In particular, to stop the pulsing when he has hazard flashers on, he may need a Pulse Preventer. Here is some text from my Reese trailer brake control user manual:
“When Towing (in most applications) with Hazard Flashers on the Digital Display will flash with the Hazard Flashers. If the Brake Control is set aggressively pulsing may be felt in the trailer brakes. Installation of a Pulse Preventer will isolate the brake control from the flashers and eliminate the flash/pulse situation.”
He may also want to adjust aggressiveness and power settings on the controller to address the “sometimes works” problem.
For better advice, go to a trailer and hitch shop, not a car repair shop.
I hadn’t thought about the brake control unit. I was thinking his electrical connection might not be wired properly. If it wasn’t wired properly, the electrical trailer brakes might be getting power from the brake lights. He didn’t say if they were surge electrical brakes or if he had a brake control unit. So he may not have had a brake control unit at all.
The Pulse Preventer to prevent this condition is available from Quality Trailer. Their email is info@qtrailer.com. Their part number is 2180S. Some of the newer brake controls have this built in also. Quality Trailer has locations throughout the US. To locate the nearest branch, call 1-877-787-2453.