Last time I drove across the border at Mexicali last April, the Mexican Customs agents didn’t even bother stopping me. All you have to do is drive through the “Nada Para Declarar” lane and you are in. I could have filled up my trunk with AKs and nobody would have done anything.
However, going from Mexicali to Calexico, CA, you have to wait sometimes for an hour, while every form of beggar/street salesperson is pumping you for pesos. And, there is a damn good chance your car might get inspected....
just come in on foot, nobody will stop ya...