Upvote:1
Unlike at most borders, the road doesn't automatically take you through passport control, meaning you drive through the border unchecked.
You can obtain an FMM in advance HERE
Or, alternatively, locate an immigration office shortly after crossing the border, where you obtain the FMM and passport stamp.
And yes, you do need it for staying more than 72 hours; however unless running into a police check, you can get away with not obtaining it in practice - as again Mexico doesn't operate border checks at the border with the US.
Upvote:3
From the Baja Bound Site:
How do I obtain a tourist card (FMM)?
Most tourists who drive across the border obtain their tourist card from an immigration office at the border crossing. They are also available at border zone airports, airlines flying to Mexico, travel agencies and other ports. Tourist cards are no longer available at Mexican Consulates. Keep in mind that if you obtain your tourist card before crossing the border, you will still have to stop at the Mexican immigration office at the border to document your entry date and to have your tourist card stamped by immigration officials.
So you will get the card at the border crossing.