(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)
If the problem were solved by a little cash here, a little cash there, bribery, there wouldn't be much of a problem. In the US there is this romantic view of how logical bribing the Mexican police is. Sometimes it doesn't work that way.
The issue is it's a total crap shoot. If the guys pulling you over had a bad week, or you seem like you are from a rival gang, or have just done business with a rival gang, or are wearing a fancy watch, they don't like you. And guess what; it's kind of funny watching you suffer.