Your going to have a hard time finding the harnes.
You might try contacting our fourm sponsor Levittown Ford but I think it unlikely that one is available.
Another option is a wrecker yard that specializes in Mustangs. Or maybe EBay. Those will be hit or miss.
Were it mine I'd get some bare crimp connectors, some marine grade heatshrink and some 3M harness tape and repair the existing harness myself. I would use bare connectors and heatshrink over crimp-n-seal connectors because they make a smaller/thinner repair ans since you probably have several wires to repair it will make the harness more manageable. There are tutorials all over the YouTube. Or find a competent shop to do it for me.
One thing I would never do is take it to a dealer. The only thing they are marginally good for is warranty work and recalls. Even then it's questionable if work will be done right...
Yup, the manufacturers are designing their support to only last 7-10 years. And, our genii are busily creating NGO’s to create regulation and mandates to put the aftermarket suppliers out of business. I recently had to have my wife’s car repaired at the dealership because I was recovering from a knee replacement. It had a bad injector. So I had them replace them all and install new plugs because I knew that I would not be able to do it for several more months. The car now has a slightly rough idle ( I suspect an intake leak ) and the experience has convinced me once again to keep those “ASE” hacks away from my stuff. Not to mention that it also cost me $2k. F the stealerships!
Soldering is not your best option for repairing a wiring harness. I mean it works but the heat weakens the wire and the hard solder makes it more prone to breaking if there's any movement of the joint. The only place any auto manufacturer uses solder is attaching components to printed circuit boards. Everything else is crimped.
But I don't blame you for wanting it to be OEM. It would bug me to on a pristine car.
If you want a new harness your going to have to hope for a dealer that is willing to search for "new, old stock". Again, I'd try our sponsors. We have two. Granger Ford and Levittown Ford.
Since you have a part number you might want to start trolling EBay and cross your fingers. Or, again, call the dedicated mustang wrecker yards. There are several around the country.
Good luck.