Waterproof shrink solder connectors?

They are not great as they eliminate the crimp connection.
And as people have said, you need hi temp wire really for that type of thing.
Teflon is best.
Aircraft stuff.

For under the hood, I use a naked barrel with plain rosin solder and some heat shrink with sealant inside it to finish it off.
Takes a little more effort but for old auto wire it works better.
(How much are you going to be doing anyway.)
Aircraft is tinned wire.
Plain old automotive copper tends to be oxidized and the solder in those crimps may not flux enough for a good bond too.