I found this post that might also help you.
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I found this very interesting article in
http://rollinghomes.com/Madds3.htm
December 5, 2005 Update - We received a pleasant shock when we crossed the border at Nogales on November 30. Motorhomes were being issued 10 year vehicle permits. The cost was 530.64 pesos or about $50.50 US. The girl at the Banjercito said that new rules went into effect three days before. This apparently does not affect pickups since we (in a pickup camper) could only get the normal 6-month permit for a price of 318.38 pesos or about $30.30 US. We'll have to see what they do with fifth-wheels. Let us know. Also, we were told that as part of the new rules one driver can now bring in two motor vehicles. That means single RV motorhome drivers can now bring in a tow vehicle. This is all new and seems almost too good to be true, let us know what happens when you come in. Send us an email using the link at the left or to
rollinghomes@pocketmail.com.
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Another
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When you arrive @ km 21 you simply ask for a 10 year permit. This is NOT part of your car permit. The purpose of this permit is to allow you to leave your trailer in Mexico for 10 years. When this program started it was for 15 years! Boats are also eligible for the same permit.
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Maybe that will help you. I suggest that you also post on Thorntree the questions and I bet you get more answers there.
cya,
shoe