make sure you get MARINE butt crimps with shrinkable insulation that has meltable glue inside. and yes, use a hot air gun, NOT a torch to shrink it, as a torch is TOO hot and will char the insulation before its properly shrunk and melted onto the wires.
you usually won't find these at an auto store or the local hardware store. Either a marine supply, or I've been using these noname chinese ones that seem to work just fine...
https://www.amazon.com/gp/product/B01DPS3ZBE