Yikes, I did a really quick check of Expedia, Yahoo, MapQuest and a general Google search for driving directions, Mexico, looked at some of the websites and wondered if I would rely on them to send a boss or coworker anywhere. (nope)
Here's what I was able to come up with, and also in this mornings "net travels" came across some very stern warnings about driving at night just something to pass along.
http://www.maps-of-mexico.com/driving-distance-chart.shtml which at least tells you how far from place to place, but doesn't provide directions, although with a little effort on the site you can also find some apparently older mapquest maps of specfic locations,,,,
Clicking on one link ended me up at EBay---Oh yeah now
THAT's helpful.
I also came up with
http://www.virtualmex.com which might at least give you someone to call to get info. And I know it's a silly idea but have you tried AAA? It's possible they might have maps...
Colour me ...Stumped!
Andrea