Can I use auto primary wire to add a new receptacle in my garage?
I would extend the circuit for the additional outlet using approved mains wiring. Being that the existing uses 14 awg wire use the same type to hook up the new outlet. The cost of the correct style of wire will not be that great. 

AC mains wiring has agency approved style insulation on the wiring designed to properly stand up to the voltage and abrasion tolerance needed for the application. Typical automotive wiring may not meet these requirements. I do hope that your existing 14-2 wiring is actually 14-2/wGND.  

Another consideration is that the solid mains style wiring works much better under the screw terminals on switches and outlets than stranded wire. In automotive application the stranded wire is used because it is more flexible and works very well with crimp style terminals.