Waterproof shrink solder connectors?

This doesn't answer your question but anything that will waterproof a splice (soldered or crimped) would be a good idea.

Back in the day I had to replace a wiper motor on a Ford. The replacement from Ford used a different connector so cut and splice was required. in the supplied kit was small pieces of heat shrink tube that had a substance inside that created a waterproof seal after it was heated. I have never looked for it but I know it exists.