If splicing a length cut all wired different lengths to avoid the spice budge. you can then use self vulcanizing rubber tape to cover the connections. This will bond and keep the water out.
Then cover the rubber tape with Vinyl electrical tape after .
I like 3M tapes. To start the vinyl tape start a 1/4 in and the wrap over the end and all the way and back up keeping tight pressure.
I if was doing it i would solder the wires use water proof heat shrink (has glue inside it) then wrap with vinyl tape. Need at least 250 watts (or more) on an American Beauty solder iron and good Kester solder.