Here's a wire soldering tip that's useful for every joint, but especially handy for hard to reach joints.
Use a technique called the Western Union splice. Look it up on Google.
Wire joint is mechanically locked together before soldering so as long as you can get the iron in there you can solder it. You don't need to hold it as it won't come apart.
With a bunch of practice and a couple of burns you can solder a joint with one hand, upside down in the underside of a dashboard as you lay in filth.
Boy I miss being in the trade, lol.
I will also add that the Western Splice also leaves a very smooth easy to shrink tube joint. Not a big ugly blob.