It is indeed a can of worms !
Soldering connectors in automotive uses is a bad idea because solder, being soft, can migrate/move about. When fresh it will give the best electrical connection but mechanically it's not ideal. It gives complete protection against corrosion at the joint because lead is, well, lead and that's why we have it on our church roofs if the pikeys haven't found it yet.
Crimping gives a really good mechanical connection (or should if done correctly) and initially gives a good electrical connection. A perfect crimp connection is also impervious to corrosion at the joint because there is simply nowhere for the water and oxygen to get into. In practice this isn't always the case because a "perfect" crimp is not easy to obtain even with hydraulic crimpers.
Crimping then soldering (my preferred method), to me, gives the best of both worlds. You crimp the joint first to give mechanical strength, then you solder the joint to give the best possible electrical joint and at the same time providing corrosion resistance as the solder both wicks into any space left in the crimp and forms an impervious seal around the joint.
Never ever solder then crimp a joint as that WILL give the worst possible joint !
Some people seem to think that soldering after crimping somehow takes away from the crimped joint instead of adding to it. I think, rightly or wrongly that at worst it achieves no benefit and at best it can improve the joint.
Over to Bo...