i have repared counless wireing looms in various vechicels and modifyed quite a few as well, and corrected plenty of modifactions by other people.
main reason for FAIL is scotchlocks closely follwed by crimps.
most crimps you find in motorfactors are crap and so are most crimping tools. my prefered way for headlamp loom would be to solder most of the conections and seal the relays in resin.
i've had hours of fun fetting around under car bonnets removeing corroded crimps