I've been rebuilding mine for a while, trying to find a used one with no damage is very difficult. New ones are over 800.00! I've found that all the used ones I have purchased been overloaded. Check the main power connector to see if you see any discoloration around the main power pin. If you do check under the fuse block for melted wires. I bought 4 used ones and everyone had some damage in that same spot there. The wiring for the mustang can not handle to much extra being added. Halogen headlights are notorious for taking out switches and wiring.
I'm currently wraping mine with some stuff I heard about from a fellow car guy. It's called cloth loom tape and it's awesome stuff. Makes for a very nice wrap and no ugly electrical tape adhesive. You can get it cheap in ebay.
I would also suggest that you purchase the Scott Drake wiring book, best money you can spend. Believe me trying to figure out that loom on your own is awful, Also putting a new box on an old harness is no easy task. The original wiring is not long enough to go through the firewall so it means more splicing.
If your going to rebuild this one the easiest way is to get a piece of plywood and lay this one out on it and tack it down with the tape removed. Then go wire to wire and inspect the whole thing. You might find (like I did) wires that were in the tape that are melted together and shorted. If you need to replace a wire with loom tacked down it's easy to trace and replace. You can also purchase a pin removal tool if you need to replace pins in the power block and you can also get the proper crimp tool with the original style crimp on pins. If your wires are bad at the fuse block you can even get new fuse holders to rebuild the fuse block.
Hope that helps.