Soldering, assuming you know how to do a telephone splice, is the best. The self sealing heat shrink would be the best "sealer/weatherproofer". Since that's not readily available, I have used RTV sealant to coat the exposed splice, slid the regular heat shrink over it and shrank it down. Need to be careful on the type of RTV sealant one uses because the ones with acetic acid are know to cause corrosion. (In Accordance with the Navy's "509" & "540" manuals.) I use Dow Corning RTV 3145, when I can find it. If not, some of the copper sealants sold at the automotive stores have less acetic acid in them.
The "509" manual is for aircraft corrosion control and the "540" manual is for avionics corrosion control.