You got possibly a small clue today.
While the motor was on the key is on.
The key being on did two things I imagine.
1) the key being on, motor running or not, probably energised your ABS hot wire in the bigtail.
2) the key probably caused your brake lights to come on.
Securely chock the truck, key on, check that pigtail wire for juice.
Now, key still on, release parking brake. Brake lights go off as if you're moving down the road ? If so, check wire in pigtail again.
I would think this may help determine if brake lights, or key in on position, is contributing to problem