I like to use 16 and 14 gauge for most of my wiring work. 18 just seems too thin and more likely to break from getting snagged on stuff, like when I cram my hand into a tight place when one of the wires happens to be there.
Inside of the cab, I usually just butt connect the wiring. I use electrician pliers to crimp them though, not those rinky-dink wire crimpers. Outside of the cab, I like to solder, but sometime use butt connectors and tape them up with some high-tech friction tape that bonds together to form a watertight/airtight seal over the connection.
Doc