the problem with soldering is you ceate a section of wire that is effectivly one fat singel strand of wire, and is prone to fail over time, the other problem is the flux residue it encorages corrosion

for circuit boards solder is optimal for wireing looms it isn't but its not the worst.
scitch locks are the worst as they arn't sealed and they also pinch the strands of the wire after time the electrical connection often fails ether due to the strands makeing up the core of the wore snapping or becomeing corroded.
crimps

this type of crimp is crap. nigh on inpossible to get enough pressure to securely hold the cores but usefull for quick tempoary repairs

these are the crimps are much better the ones you really want have wings that are folded round by the crimping tool and forced down into the cores of the cabel

they look a bit like that and are the best way of joining wires in automotive applications once coverd in heatshrink.
thats how motec reccomend you join wires if you have to when installing there ecu's tho the ideal is to replace whole wire it's how vw train there monkeys to repair looms and the crimps that hold all the wires into your ecu plug are the same as that one the ones in your fusebox ain't much diffent ether