2008 T660 Trailer brake lights don't work

It might be the pig tail. You by passed the pig tail ao you didn't check it? We had a truck do that and pug tail was replaced and it worked. Sometimes the water gets in there

 

You honestly don't have that kick panel under your dash next to the ABS diag switch?

 

There's a relay for the trailer brake lights. When you hit brakes, a 12v low amp signal travels down to a fuse, from that fuse it goes to the relay and closes it allowing a 12v high amp current to travel back to the trailer.

The silver things behind the kick panel aren't fuses or relays, they're circuit breakers. The fastest way to test one is to swap it with a known good one.

 

Power flow starts at the brake light switch, and goes to "Trlr Stop" fuse (mine is a circuit breaker) then it travels to the "Trailer Stop LP Relay"

Give me a sec and I'll tell you how to bypass the relay and fuse with a short piece of wire to tell if the issue is related to your fuse panel or somewhere else.

 

Ok, hopefully your fuse panel is similar to mines. Pull the trailer stop relay and do what I did with the white piece of wire.

You're going to bypass the fuse and relay and close the high amp 12vdc circuit manually which is supposed to turn on just your trailer stop lamps.

If they come on, then your issue can be narrowed down to your relay, fuse and anything before it. If they don't come on, then your issue is after your fuse, and relay, and will be in that rats nest of a wire harness after the fuse panel all the way back to the tail lights of the trailer.

You can isolate the trailer by putting a ground on the pig tail (top white pin) and applying 12vdc directly to the red cable (I want to say it's the bottom pin). That will eliminate the trailer and pigtail as being the possible cause.

Then you'll only have to check the harness coming out of the fuse panel that runs behind your washer fluid container.

But start by jumping that relay.

 

That would indicate that you do not have a high amp 12vdc coming in which means something is disconnected behind your fuse panel.

Use a light tester or volt meter to probe the relay points just to double check.

87a is a ground so it won't show any current.
30 goes to the brake lamps so it won't show any current.
87 should show 12vdc
And 85 or 86 will only show current when you hit the brakes.

I'm pretty sure based off of what you're telling me, your 87 will be dead, which is sorta a good thing cause now you know what the issue is.

If your 87 is actually dead, then as a temporary fix, you could run a jumper wire from a spare 30amp fuse (not a circuit braker) down to your 87 (just as you did the 30) and everything should work normally and the circuit would still be protected from a short. It'll buy you sometime until you can get the washer fluid container off and start digging around in the back of the fuse panel.