On the Mexican side of the border, checkpoints are typically staffed by Mexican Army soldiers or Federal Police officers. They may ask for your passport and vehicle registration, and they may also search your vehicle.
It is important to be polite and cooperative when interacting with Mexican authorities at checkpoints. If you are asked to step out of your vehicle, do so calmly and without resistance. You should also avoid making any sudden movements or gestures that could be misinterpreted as aggressive.
If you are unsure of what to do at a checkpoint, you can always ask to speak to a supervisor. You can also contact the US Consulate in Mexico for assistance.