Welcome to JA
If you have a 5 wire brake system, check your grounds, white wire on trailer
and check the blue wire (brake solenoid)
if you apply 12V to blue wire, and you here a clicking, you know it works
if you have a 4 wire system and a surge brake, it sounds like you brakes are bad or the surge actuator is going out
Marine Mechanic
I HAVE BEEN WORKING ON BOATS OVER 25 YEARS