1) If there was water coming in the grommet aka boot, you need to seal that up better. One option for sealant is sensor safe (non-acid cure) black RTV, available at any auto parts store.
2) I've linked a youtube video showing how the water can run down the wires, and one solution was to elevate the connector and have droop in the wire before the connector, so the water running down, drips off before reaching the connector, and the video suggests once the connector is making good electrical contact, put some silicone dielectric grease on the contacts to help keep water from causing damage. If you end up cleaning corrosion off the contacts and then they are bare metal (steel or brass?) then the dielectric grease will also help keep oxygen away so the bare metal does not oxidize, since cleaning the corrosion off may strip away any plating they previously had. Obviously #1, sealing the entry point is even better but this redundancy should help. Brasso or other metal polish with oxalic acid in it, is good for cleaning corrosion.
3) There is a direct link to a PDF of Ford connectors linked below in my sig, you might look through it and see if any match what yours is, then you have a part #, or can the shop give you a part #? I would only expect 2016 to 2019 Explorers to have the same harness, otherwise other vehicles using the same, would take the same part #, so having the part # may be important, or search some more junkyards to pull what you need. There is also this website, somebody on there is going to have it, but the tricky part might be how to search for that. You might search for other liftgate parts, since having any would mean they're pulling parts from an Explorer liftgate, and it includes the yard name and phone #:
4) What is the extent of the damage? Is it only on the harness wire or connector to the camera, or the camera's connector and/or wires too? Often the connectors have an insert that can be removed, then the electrical contacts pulled out, possibly cleaned off, and if the wire is bad, cut out the bad portion, see if you can either gently pry the crimp on the contact apart and re-crimp to fresh wire, OR smash it flat and solder fresh wire to it. If the original harness wire is too short after removing a damaged area, just add some wire. Remember that soldering it would mean using minimal solder so the contact-wire solder joint fits in the connector cavity. If necessary to make it fit, you could add smaller diameter wire, as this camera does not use much power at all.
4b) If it's just one or both of the connectors damaged, you might be able to cut the connectors off and direct solder the wires together?
Pictures might help if the above does not address the problem.
I looked through a 2014 workshop manual and it lists the connector at the camera is part # 14A1412 but it seems that this is not enough, that this part # is also part of the longer part # for multiple vehicles that have different harnesses. It does not list the harness to it, at least not that I could find. That workshop manual is also linked below in my sig.
Lastly here is a used one on ebay, but ebay shows it is already in two (peoples') carts, so if you want it, you might need to buy it right away, or look elsewhere on ebay. I found it searching for Explorer camera harness. The seller claims it is part # GB5T-19G490-AB but whether that is true and for the whole harness section or just one of the components on it, I don't know. That is possibly just the part # for the camera itself which the workshop manual shows is 19G490.
A few other things that I noticed but cannot confirm, are "probably" a '17 harness is the same and one ebay seller lists it as part # HB5T14A583AB, and the connector at the camera itself might be part # 9U2Z-14S411
It is now 3:14PM EST. If you read this before then, I've added a lot more to the post.
