All I will say about soldering is that although it is said to give the lowest resistance and most reliable joint,it isn't always the best way of joining two wires together.Crimping done properly under VERY high pressure will form a cold weld that will melt all the parts together and give a much better joint than soldering ever will.
Manufacturers will never solder inside a wiring loom,partly due to manufacturing costs,but also because a multi-strand wire that has beeen soldered turns into a much more brittle single strand wire,that is more prone to failure.
It is however the best way for the small scale amateur messing about with low current lights etc on his/her own car,the mistake most people make is either not enough heat (you get a dry joint and improperly melted flux can make a worse joint than twisting the wires together),or too much solder travelling too far up and down the wire,also caused by too much heat.
Chris.