I never would drive south of that border without 2 good forms of ID for the American side. For the Mexican side I take at least 500.00 in 20’s, split up and hidden and a copy of my border insurance. If going to Mexico I’d highly recommend a rental with Mexican plates.
A friend of ours (kids' Godmother) just moved back here from Mexico. She was held up down there twice (by the SAME GUYS!), once while driving and then at her house (they evidently paid off policia to look up her MEXICAN license plate to find her address). She was sure she was going to die during the first holdup and lost a lot of money (she won't tell us if they had pointed a gun at her -- I think they did or something even worse). Apparently not much happened during the second hold up partly because she had junk cars parked outside her house to make the family look poor (a trick often used in third-world countries). So I would say having a Mexican license plate doesn't make you particularly safe down there. Maybe a Mexican license plate on a junk rental car is the way to go.