2006 trailer.......
Get a magnetic compass
Hold compass close to wheel
Have someone operate trailer brakes via manual lever
Watch compass needle.....should deflect towards wheel when brake magnetic gets energized
Compass deflects at all wheel positions.......brake magnets are at least getting voltage
Next thing is to do a manual adjustment on brakes
Any that don't or not as strong of a deflection.......that wheel position has wiring issues
Most likely at wire connections going into brake drum or wires going thru axle tube