OP I have over 40 years of automotive experience under my belt. You want to join a 10ga to 12ga wire and don't know how to solder. These are probably stranded wires you are encountering as I believe you said 12volt. Use a butt connector and a good crimp tool and you will be fine. Tug on the wires after crimping to make sure they are secure. If it comes apart redo it. If it is exposed to the weather cover with a piece of heat shrink tubing. A good idea to use it anyway. Wire nuts are not generally used in automotive applications for 12v circuits.
