Two stories:
1. My former 02 Continental never had the cabin filter changed until I decided to check it at around 85k miles. Filter was located under the hood, under the right side of the windshield. Managed to get it out, and yes, it had 85k miles worth of dirt and leaves in it. (BTW, it was not blocked, and air flow was good). Tapped the filter to get the large leaves and dirt out, then took a vacume cleaner hose, and cleaned it out good. Replaced it. (too cheap to buy a new one). Was still working great when I sold the car at 96k miles.
2. My 10 SHO with 30k miles on it. After much looking, I finally found the cabin filter, up and behind the glove box. Removed the filter, and sure, there was a little dirt in there, but not enough to worry about. Again, took this filter and tapped what little loose dirt out, being very carefull as the filter is very flimsy. Then I vacumed the filter and replaced it. (like I said, I'm cheap).
Have not checked the filter on the 11 MKZ with 9k miles, but I'm sure it is also up behind the glove box.
Works for me.