By car
If you follow International Blvd. south out of Brownsville, you'll cross over a short toll bridge ~US$2 and immediately be on Av. Alvaro Obregón. Border customs and security in the U.S.-to-Mexico direction is light, at worst a cursory glance-over, after which you'll immediately find yourself plunged into the streets and traffic of Matamoros.
If you're only planning to visit the "border zone," an area extending roughly 25 kilometers 15 miles south from the border, you won't need a vehicle importation permit or a tourist card. If you intend to venture farther into Mexico, however, it's easiest to obtain your vehicle importation permit first thing at the border. Right after crossing the international bridge literally at the foot of the bridge, you'll see a white building on your right housing all offices you may need to deal with: the Mexican Tourist office, Banjercito branch, and several Mexican automotive insurance Seguros de Autos vendors.
