Don't solder anything that will experience vibration or flexing. The solder makes wires brittle and will break. If you notice from the factory, the only things soldered are things that connect directly to a circuit board, etc. and have some sort of strain relief to protect the soldered area from flexing/vibrating
I think it should be noted that properly splicing wires using solder and an adhesive heat shrink is far superior to using a basic crimp with automotive store tools. Lineman splice is the way to go
See page 71 of attached Nasa STD 8739.4a