Typical sealed junction, in this case in the red/white instrument illumination circuit
Brass 'staple' crimped round the wires and soldered ...
... with a heat-shrink end-cap plus a length of standard heat-shrink tubing over the junction
After-market solder-sleeves in white, red, blue and yellow for various wire diameters. A ring of solder in the middle for the electrical connection and two areas of adhesive either side of that for mechanical strength and sealing, for use with a heat-gun. I had wondered if the solder was a special low melting-point type but standard solder melts with my heat-gun as well:
Strip the insulation - the conductor strands need to be shiny copper for the solder to flow:
Fit the sleeve over one of the wires and push the stripped ends together so the copper strands interlace. Twist the two wires slightly to bring the strands together without pulling them apart, and slide the sleeve with the solder ring over the conductors:
Apply the heat gun to the solder ring and after a moment you should start to see the solder flow, shrink the sleeve, and melt the adhesive:
It's not perfect, while solder has flowed over some of the stands it hasn't over all of them, and the insulation at one end of the sleeve shows some heat damage, it needs a nozzle on the end of the gun to focus the heat. I tried a soldering iron but it didn't get it hot enough.