When getting a Cancun rental car, be sure to ask about the license plate.  If you park illegally in Mexico, the police will take the license plate.  You will then have to go to the police station.  In order to get the plate back, you will have to pay a fine.  If you return the car to the rental company without a plate, the company will charge you a fee.  The fee is to cover the cost of having to track the license plate down.  If you see that your rental car does not have a license plate, ask the rental company about it before driving the car away.  Some cars do not have plates but only a paper taped to the back window. 

You should also be aware that the gas station scam is somewhat common in Mexico.  Keep in mind, when getting rental cars in Cancun you will probably have to fill the gas tank at some point.  Gas stations in Mexico have attendants.  Some attendants will fill your car without clearing that last purchase from the pump first.  When you pull up to the pump, quickly get out of the car and make sure that the pump is cleared to zero.

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