Got certified in automotive electronics 15+ years ago, and having lived in Wisconsin's harsh salt-infested winter road conditions as well as Texas' salty air, the best way that I've found so far is NO SOLDER with the GOOD 3M tape ($3-$5 a roll). Here's the theory (that has worked for well over two decades for me):
If you use crimps/wire-nuts/etc., you have created a point in the metal (wire) that does not flex with the rest of the wire. We all know what happens when you keep bending metal back-and-forth after a while - it breaks. So that's why I don't solder - the wire remains flexible even through the junction. Now wrap the tape around it as tight as you can, and do two layers - one that just overlaps the junction a little, and then wrap over that with a second layer - in the opposite direction.
Doing it this way keeps moisture out (so as to avoid corrosion), and keeps the joint flexible so it doesn't break under constant flexing.
There's no concern about the junction pulling apart, because you've wrapped it twice with quality tape. If you're pulling it *that* hard, you've got other issues.
If you've got a plug of any sort (like where the harness connects to the trailer), put a little Vaseline in there to keep moisture out. It won't short (trust me), and it's not something you mind getting on your fingers in the event you grab it wrong. I've seen people use axle/bearing grease for this as well, and while it does a good job, it's not as easy to deal with as Vaseline.
