Before you cross the border, be aware of the signs telling you where the declaration lanes are if you need to declare anything to the Mexican customs officials. If you need a vehicle permit or a tourist card, you may drive thru this lane as well, park, and obtain those documents.
If you don't need to declare anything, you may cross in any lane that says "Nothing to Declare" (Nada que Declarar). But this doesn't mean that you won't be questioned or searched by customs officials. When you cross, you will see a traffic light with a red and a green light. Red means you have been selected for secondary inspection and green means continue on your way unless an official tells you the opposite. If you are selected for secondary inspection, you must pull over into the customs facility. There, they will conduct for the most part, a quick search unless they find something that should have been declared. Usually they check the trunk and take a glance inside the vehicle. A kind and gentle behaviour is always appreciated by the officials.
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