Well, the beheadings impressed them so much that they decided to take it to the next level.
The idea of thought behind who taught them is dictated by patterns and trends. In other words, who else cut off heads and use car bombs? Let’s start with the obvious first: the vast majority of Islamic terrorist groups. Therefore, one could easily postulate that the Mexican cartels are either copying or incorporating the techniques of Islamic terrorist groups or they are being literally taught these varying terrorist methods by actual Islamic terrorist groups. This then leads to the various reports inb recent years citing the growing convergence between international criminal groups/organizations and varying Islamic terrorist groups, specifically those reports directly aimed at the convergence between the two largest Mexican drug cartels and Islamic terrorist groups.
Example:
http://www.conservativecrusader.com/articles/al-qaeda-and-farc-the-unholy-alliance
http://www.washingtontimes.com/news/2009/mar/27/hezbollah-uses-mexican-drug-routes-into-us/