Interesting.... I know I looked at my ST and there wasn't one, I Know on my moms sonata you can insert one in the car, the slot is there... I wounder if the St is like that??..
Andrew, Can you check your owners manual to see if it says any thing about your ST having one???
sounds like the dealer charged you for something that was not needed....tell the dealership to point it out to you, show you what they did because The ST's don't come with one out of the factory....I've gotten confirmation that they don't from a Ford Tec in Eagle Pass TX and also from one in Austin TX and there is no cabin filter in the my or any of the other ST I've looked at. If they didn't show you the filter come out of your ST, tell them you're going to file a complain with Ford and hit them up with a police report for false charge of service not perfomed and taking your money.
on other vehicles the cabin air filter is usually under the wiper cowl. it is from what i have seen a round filter in which you can fit your arm through the center......maybe a foot long and 7 to 8 inches in diameter.....again this was NOT on a ST.