Trailer brake lights continuously on

I would've said its your brake light switch stuck on, but you said it works normally without the key on so its probably not that. My guess is the brake light circuit is somehow getting power from the ABS wire (center pin in the plug). I'm not sure, you might try pulling the fuse out of the ABS in the fuse panel and seeing if that does anything.

 

Check to make sure the Trolley bar isn't pulled out a little....
It only takes about a 1/4" for mine and the brake lights are on....
A 1/4" won't cause the brakes to drag...but it's enough to short across the switch.

 

Are the brake lights on the tractor OK or are they on also? If they are alright then I would try swapping trailer light cords with someone else and see if that fixes it you could have a short in the cord.

 

try pull up and back up, you might not be getting a good ground on 5 wheel. without good ground you can't tell much about lights.

 

the center should be ABS and the big terminal ground .does the trailer have ABS ?? if the cord ground isnt making contact feed back can occur in the lights

 

are the lights on the trailer LED
if they are your prolly getting enough power down the blue wire to light up the red wire
there is only power on the blue when the ignition is on
look for green corrosion on the back side of the plug on the truck and also on the backside of the plug on the trailer side (female sides)
with LED's enough power can get through the corrosion to light up the lights