I've used liquid electrical tape, I use it in my preferred method of joining wires together. When I needed to add my kill switch I would splay the ends of the wires together, apply solder and have it run between the wires for best connection, and then I would use liquid electrical tape to waterproof the solder point. It is a reliable and long lasting way of doing things, but the liquid electrical tape is messy because it never stops dribbling so you have to put something underneath the wires you will be using it on.
Another way of joining wires together is a spade connector (seen above). It is often used to join wires to the back of switches (OEMs use it too), but you can also use it to join two wires together. The downside is that you have to crimp it, which can take a bit of practice, and the connection isn't waterproof (there are spade connectors that use heat shrink but they aren't as common).
The last connection type commonly used in automotive electrical use is the butt connector. These connectors come in two variety, crimp or solder, but they accomplish the same thing. The butt connector is a tube of heat shrink with some way of joining the wires in the middle.
On the crimp kind (seen above) you have to shove the bare copper end of a wire into a metal tube in the middle of the connection, you then use a crimping tool to crush the metal tube down onto the wires to hold them in place an make a connection. You then heat the heat shrink to make the connection waterproof and stronger. I am not the best at crimping so I try and avoid this kind unless necessary.
The solder kind of butt connector is simpler to use, you push the 2 exposed copper wires together in the middle where there is a ring of solder, as you heat the heat shrink the solder will liquify and join the wires together. I don't use this kind because you cant make the solder flow in-between the wires, so the result isn't as ideal (this point is illustrated in this
video).
Whatever connector you prefer is really a personal taste. All that matters is that in insulates the wires at the connection to prevent an unwanted path to ground, is reliable enough to hold up to years of vibration, and it should really be water proof. I discourage the method of twisting two wires together and using electrical tape to insulate them, its lazy, unreliable, and not waterproof. Not to be confused with wire wrapping on a PCB, that method is very sound but sadly now obsolete (below).