This last comment from MEXICOWANDER is true, I have a friend that got his car stolen in Mexico City and took close to six months of doing paper work and a lawyer for him to be able to just leave the country, Another friend of mine and I rode with bill, at the famous check point before reaching Nuevo Laredo he was detained because he did not have the vehicle, he had all the papers from Hacienda, Aduanas, Relaciones Exteriores, Police etc, two hours later and after having signed over the stolen vehicle to the aduana one more time they let him go.
take photos of the vin number with your passport next to it and keep them safe, also if you can the tip decal on the windshield.
The VIN " Vehicle Identification Number" number is on the dash board left corner and is visible through the windshield.
navegator