When splicing 12-volt wires in your boat, a super-strong, watertight soldered connection beats crimp-on connectors hands down. You’ll need wire strippers, heat-shrink tubing, a soldering iron, and rosin-cored electrical solder (not plumbing solder).
Strip about 1″ of insulation from each wire, and slide a length of heat-shrink tubing over one wire a few inches away from the connection. Unravel each wire end, push the strands together to intermesh, and twist tightly. Or, twist the strands on each end tight, then overlap and twist tightly together, spinning one set of wires around the other. Make sure to tuck in any loose strands.
With the wire supported by a vise, a clamp, or something similar, heat the bottom of the joint with a soldering iron until the solder melts and flows when applied to the top. (Heat the wire itself, or the joint will be weak.) Move along the joint until it is smooth and full of solder. Let the joint cool, slide the tubing overtop, and shrink the tubing with a heat gun or a lighter. Your two wires are now one.
Don’t use too much solder; you should be able to see the original shape of the wire.
A helping hands tool will hold small parts in place while you solder.