Yes, I was thinking the same as you are.
But, . . from the broken bolt the previous owner took ground for the trailer.
After the chassis swap I reconnected everything 3 times the way it was (except the black wire, I took ground from the stud for the mudflap) and now the trailer brake lights don't work.
I changed the ground to the chassis but same result.
Also when I measure current on the brake light wire there is.
As soon as I connect the plug to the trailer all current is gone?
My guess is the wire does something.