RFC's are issued at SAT Offices. SAT Iis located at Av. Francisco Villa 1292, Las Aralias, 48328 in Puerto Vallarta.I
However, I am trying to figure out why you need to have a RFC to register a new vehicle, unless you are in/or have a business in Mexico. If you are just purchasing a vehicle for private use, the dealership should be able to issue you a factura without you having a RFC, (which can be considered the "official invoice" (BTW once you get the factura for your vehicle, guard it with your life. Without it, you will never be able to legally sell the vehicle in Mexico, that is to say, the new purchaser will not be able to register it.)
In order to get a RFC number, if you must, you must have a CURP number assigned to you (CURPs are basically a form of ID needed for various legal activities and transactions in Mexico). CURPs are processed at immigration offices, referred to as INM in Mexico.
Mexicans love acronyms!
If you still are unsure of these things, find yourself an English speaking accountant in PV.
Hope this helps.