I don't have the info saved anymore, but last year I looked into driving from Texas down to Tampico. One thing I do not see mentioned in this topic so far which I found in my reading is that if you bring in a car into Mexico, you have to pay a deposit on it, as a guarantee you are going to bring it back.
Oh, I just googled it for you and found the info again...
http://www.mexonline.com/drivemex.htm
Step Three: Pay the certificate fee and bond deposit
Pay the certificate fee (not returnable). At the border, your international credit card will be charged an amount in national currency equivalent to US$27.00 to the Banco Nacional del Ejercito.
Pay the "guarantee return of the vehicle to USA territory" bond via credit card, according to the schedule below.
Model Year Bond Amount
1999 to present US $400
From 1994 to 1998 US $300
Models older than 1994 US $200
You'll be issued your documents, which completes the process for acquiring the certificate. Keep in mind the full process is not completed until you return the certificate upon heading back into the United States, see below.