There are two ways to fix that, well at least two... Pull a little (on the existing wires to get some slack if in an area where you can), strip insulation and solder it back together, or the better method of adding some high strand count new wire so you don't have only the same stressed wire plus a less flexible solder joint in the area that's going to continue to get flexed every time the door opens.
Ford should have used higher strand count wire there in the first place. The typical sources/applications for short pieces of such wire is silicone insulated wire for R/C hobbies. It does cost more but you can find sellers on amazon, ebay, etc, selling a few feet at a time instead of a whole spool. I wish the local hardware stores sold this by the foot but unfortunately they do not.
Then again if the original lasted this long, just the same fresh new wire type that's cheap at a hardware store to get another several years of use might be a good compromise.
