Use a butt bonnector, crimp first then solder, wrap with some insulating tape then slide the shrink wrap over the joint. Twisted soldered wire tends to have "points" that will cut through shrink wrap eventually.
If the soldering is successful the solder will "shine" if not it appears "matt".
If I were doing it in the car IŽd rather not use a butane torch.
Even with a good soldering iron, in car soldering is a pain.
If it is in an area such as under the dash, IŽd just crimp.