In my days at Peugeot, we used to use solder crimps to repair wiring connections where plugs were failing.
They were ingenious... Basically it was a ring of solder in the centre of a in-line crimp, completely sleeved in heat shrinkable material. You just inserted the wires in each end, then used a heater ring on a soldering iron to melt the solder, then ran it top to bottom to then melt the heat shrink material.
Then hey presto, one proper soldered joint, completely sealed from water/moisture, grit and dirt.
I found loads when I moved to Ford.. It wasn't until I needed some a few years later and finally found them I realised the price - Over £1 each! I never used them again! lol.