Best way to splice into a 12 volt wire?

What Mr Wiz said. Depends on what you want.

Crimping in tube/butt joint has to be done properly, and the cheapest tool that "looks" proper, costs $40 on Amazon, not hydraulic of course.

You can't go wrong with a split bolt or set screw butt-joint connector, costs a few bucks in Home Depot, you can wrap it into insulating tape and dissemble later with just a screwdriver. For low-current 12v circuit it doesn't have to be perfect.

When I have done it, I have used crimp on butt connectors. Maybe not the best, but works, and if it comes loose, you can pull the wire out and replace the butt connector for another try.

I refuse to use 3M Scotchlok connectors as they will eventually cause intermittent problems especially when exposed to humidity. Seen way too many problems from them. They are quick & easy but that is all. :(

Cut, strip, add in extra wire, get butt connector with adhesive lined shrink tube from West Marine, crimp, heat the shrink tube till the adhesive oozes out, cool, good to go!

splicing for 'what' ?

location , wire size, and load dictate what can be done, which may not be optimum which can be done in a easily accessible location

if it is to be permanent soldering is best

next crimped with a hydraulic crimper

next use a 'split bolt' connection and tighten fully, before taping and sealing

something you might want to change

split bolt is best
next is solid 'wire nuts', not the soft scotch lock type

the only time i will use the soft ones is if that is the only thing available, and time is of the essence, then i will replace them with the good kind

Best way?

Cut the existing wire. Strip back enough to solder. Strip the additional wire to the same length. Twist the "new" wire onto one of the "existing" wires. At this point, slide on some shrink tubing of the appropriate size, and slide it out of the way. Flux and solder the 3 wires together. Using something like liquid electrical tape coat the splice and let dry. Slide the shrink tubing over the splice and shrink it (heat gun is best).