A visa is not needed for RP, but next month we plan to extend our trip to San Carlos for a week after entering at Sonoyta. Anyone know if we can obtain a tourist visa in Sonoyta, and if so, any details on the process?
You can obtain a tourist visa in the building on the right just after you cross into Mexico at Sonoyta. Proof of citizenship is required, but a driver's license is usually accepted.
It appears that you don't need the vehicle permits, but you still need the Visa to visit those areas, although I wasn't asked for a Visa at the checkpoint south of Puerto Penasco when I drove down to Caborca.
The "free zone" extends down to Empalme, just south of Guaymas. Beyond Empalme you need both a tourist visa and a vehicle permit. The check point on the way to Caborca is at the former free zone limit. A tourist permit is required if you are anywhere in Mexico for more than 72 hours.
The vehicle permit requirement is enforced beyond Empalme. I was pulled over by PFP just below Empalme because the officer did not see the permit on my windshield. As soon as he observed it, he sent us on our way.