Sure it will. Your radio won't know if you crossed the border or not.
Now the further south you go, the further from the beam you'll get and more likely it might NOT work.
It might though. XM has a geostationery satellite, which means it is hovering over the equator, which is south of mexico. So it's beaming north over mexico. So I guarantee you that you can pick it up in several areas of mexico. Like I said, the further north, the more likely.