Just like Crab said, solder and heat shrink. I use either dielectric silicone grease, De-ox or Oxyguard on the soldered splice before you slide the shrink tubing on. That way when it shrinks, the stuff oozes out the ends and you know its a water tight seal.
I recomend the silicone because you should use it on all your electrical connectors as well, like from the stator and your start stop switch.