Electrical Short?

IMO, you do not have a short. What is have is called a parasitic drain. It can be very difficult to locate a parasitic drain.

For the Mustang, the most common causes of parasitic drain are radios, power seats, and add on modifications.

For the power seats, look at the power seat switch. Is it loose, broken or stuck in one position? If so, start there. Perhaps disconnect the power seat electrical connection to see if this stops the drain.

For the radio, pull all of the radio fuses. Does this stop the drain? If so, focus there.

For all others, the usual way to trouble shoot is to connect an amp meter to the battery negative. Measure the current. Remove fuses one at a time and record how much the current drops by. Post the fuses with the largest drops.

A set of wiring diagrams would be a big help. If interested in getting a copy for yourself, I maybe able to help. PM if interested.