Some ABS systems if you unplug and replug the cord 3 times quickly, and then leave plugged in, will flash a fault code. It is usually a 2 digit number that repeats a few times. The fault code would give you a more specific idea of where to look. You may need to Google the fault code for your brand ABS system to find the codes.
I had a similar problems on one of my trucks. The trailer ABS would come on only when of the trucks connected to it. Turned out to be a fuse on that tractor for trailer ABS power. (Truck was a Peterbilt 387)
Glad you mentioned the fuse again. Thought the short I found the other day was the cause of it. Think it was part of it, but there is more. So fuse is blown again. There are some wires on the trailer that are taped up(from previous owner) that will get torn apart tomorrow morning to see if something separated or is corroded. Other than that, all the wiring on the trailer is a complete sealed system.