Baja California is considered a free zone or frontera and does not require a temporary import permit for visiting or driving your vehicles in Baja California, Mexico. Mainland Mexico does require a import permit past the border zone (25 Kilometers)..
The state of Sonora operates a program known as "Sonora Only" for visitors who stay only in the state of Sonora. The "Sonora Only" permit requires less documentation than the standard 'Mainland Mexico Vehicle Permit', and the 'Sonora Only' permit is free. (The need for a credit card is also waived). http://www.gotosonora.com/
Permits for driving into Mainland Mexico are available online at http://www.banjercito.com.mx