Price: $22.00 (dealer) or $8.00 through parts.com
Time: 5 minutes
Parts: Passenger Compartment Pollen Filter (Motorcraft part FP-38)
Open your hood (be careful if the engine is hot!) and locate the clips under the passenger-side windshield cowl, pointed to by the arrows in the picture below. Push in the clips so as to allow the cowl to lift up. Lift the cowl lightly so as not to pull out the clips in the back*. You should see the shelf for the filter under the cowl, and in its place a plastic grate. The front of this shelf swivels down, just unlatch the sides (circled). Now remove the plastic grate and replace with the filter, noting the proper side up. Reclip everything and you're done!
*If you were like me, you lifted the cowl out of its plastic rear clips, located under the plastic circles marked with a * in the picture. You will probably have to remove the cowl to reseat them. To do so, turn your wipers on and turn the ignition off when they are up, so they are in an upright position. This will expose a phillips screw (may have a plastic cover), marked with an A in the picture. Unscrew this. Now you may be able to slide the cowl back onto the clips by sliding it left, then pushing down while sliding back right. Otherwise remove it completely, use pliers to pull the white clips out of their sockets, slide them into the cowl, and carefully guide the cowl back in place with the clips snapped in. This is not easy and will be annoying but you'll get it! Also note newer Foci may have black rubbery sealant between the cowl and the windshield--don't touch it!