Reddit reviews 3M Heat Shrink Butt Connector Kit 140 Piece

I use these at work in marine environments:
http://www.amazon.com/3M-Heat-Shrink-Connector-Piece/dp/B008HMHJDO
Over that I apply more heat shrink. Holds up pretty well.

Alternatively, invest in some Heat Shrink Butt Splices

Much less messy and far sturdier than twisting and wrapping with electrical tape. You just insert each end of the wire, crimp, and hit it with some heat to seal it. This way is weatherproof, electrical tape isn't.

You don't need to slip over the connector, the heat shrink is already on the butt splice. You crimp then heat and it shrinks down and seals everything. There's a coating of glue the melts and oozes out. But you need to make sure you have a good crimp too, cheap ones will break or cut the heat shrink when you crimp. The glue also helps hold the wires in place. If you shrink it down without crimping, you'll still need to apply some force to rip the wires out.

https://www.amazon.com/3M-Heat-Shrink-Connector-Piece/dp/B008HMHJDO

1. Cut the old connector off
   
   
2. Strip about 1/4" of insulation off the vehicle end wire
   
   
3. Do the same for the new pigtail end
   
   
4. Use 3M butt splice (get the good ones that are heat shrink too) https://www.amazon.com/3M-Heat-Shrink-Connector-Piece/dp/B008HMHJDO
   
   
5. Tape up the new butt splice with electrical tape or some type of protection

These connectors are the best type to use, especially if it's exposed to weather. I even tape or shrink wrap over these sometimes but it's not necessary.

If I have time and the project is appropriate or I can't spare extra bulk in a wiring harness, I'll skip covered connectors and use solder with bare metal connectors and shrink wrap.