Mexican immigration officers will accept your US passport card if they happen to be checking foreigners that day. Most of the time they don't even bother at the border.

From the USA you'll walk through a one way revolving gate on the far right side of I-5 (heading south) just feet from the border. Look carefully on the right side for the small monument which marks the physical border. Notice how the border is painted across I-5 to your left. Keep walking straight; you'll pass by Mexican immigration and customs who probably won't even give you as much as a glance. Continue straight ahead and you'll see a large covered taxi stand and off to your right will be a walkway to the downtown tourist section of TJ.

To return to the USA cross over the highway using the new pedestrian bridge just behind the taxi stand at the border. After crossing the highway you'll be on the right side of the road. Walk north on the sidewalk into the USA and right into US Immigration & Customs facility.

If you are taking a taxi back to the US border, ask the driver to drop you off at "La Garita Fiscal". It's easier for him and you. Just follow the curving street north which joins up with the sidewalk to the US border as mentioned above. If you aren't sure, ask the driver but it is fairly self-evident once you get there.