There's a very useful item out now, especially handy for situations where you do not need the wires shortened, but making them longer isn't really a viable option either. Like repairing the rear door wires in the above picture. It's a piece of FIT-200 heat shrink, no meltwall inside, with a tube of solder in the middle. When you shrink down the heat shrink, the solder melts, soldering the wires together. Voila! Two pieces in one, with soldering the wires, and heat shrinking the join happen at once. I believe some even have a crimp tube outside the solder, but I may be wrong on that.

Soldering the join IS always a good idea, but ensuring you have a good mechanical connection is also vital to a good repair. In other words, and I learned to do this in my first soldering class in the Corps, tinning the wires, making a hook out of each one, and crimping them tight to each other, is the first step to making a good repair of broken wires. The instructor liked to stand on one end of the wire, and hoist up on the other to test the solder joins we made.

Sadly, that method shortens the wires, which is a bad thing in this situation. The heat shrink/solder tube item removes the need to prep the wires by hooking them together. No shortening of the wire. Unfortunately, the mechanical connection IS weaker, but not a lot. Just make sure you use the right size heat shrink to fit the wire correctly. It should be just big enough to go over the wire's insulation. That will be the strongest mechanical join.

Tin the wires, if at all possible, before inserting them into the tube. This will help ensure the solder join is the best possible, and will allow the solder to flow into the wires better. Also, using a good liquid flux will make the solder flow evenly, ensuring the best solder join possible.

The solder/heat shrink items are available from Grainger, and McMaster-Carr, either individually, or in kit form. Very reasonable prices.

Sorry, I ramble on this subject, having been a radar technician, and shop supervisor during my career.
Good luck to you!
Pat☺