When you go into mexico the US customs does not check. After you cross into mexico there are Mexican customs officers looking into your vehicle as you drive by. They occasionally stop vehicles and check to see if yoare bringing anything where you need to pay a duty tax. For example, if you have a pick up truck with a clothing washer they see in the back you may be stopped and charged tax.
No, the Mexican Customs just could ask to revise what are crossing from US. NO PASSPORT IS REQUIERED WITHIN 35 MILES AFTER THE BORDER.
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