Without a crystal ball any of us would be guessing. Your guess is as good as ours at the moment. The only SD wiring harness I ever bought was an alternator harness. Before I could use it, I had to unwrap it, rearrange all the wires to their CORRECT positions, and then re-wrap it in electrical tape. Hopefully your problem is not that severe but who knows. Get the wiring diagram and trace down what is happening. That is the only way to figure it out. Mustang Wiring and Vacuum Diagrams : Average Joe Restoration
Unless I was working as PR
for SD I would reserve the word 'great' for an item that at least worked as it should. Otherwise its just a jumble of expensive wires.
BTW, the fuses are sold by AutoZone, NAPA and O'Reillys. 'O' may have the best and easiest to use selection from what I saw last time I was there.