Not a Mopar But Ford Electrical Question
If it was a short that bad it would be smokin' the cables, heating them up, and producing a HUGE arc when hooked up
I say you have a cable that is internally corroded.
Hook up the battery
Check the TERMINAL voltage of the battery. By that I mean stab your probes directly into the battery POSTS
Now move down the line one step at a time
EG:
Leave on probe stabbed into the NEG post
Move the hot probe from the POS post to the battery clamp. Voltage?
Move to the stud on the starter relay. Voltage?
If that checks out, test the ground side
Stab one probe into the POS post and leave it there.
Move the other probe to the NEG battery clamp. Voltage?
Move to the ground point of the cable, the frame engine block, wherever it connects. Voltage?
I'm betting you lose it somewhere right there