Ford Repair: Cabin Filter, screw holes, cabin filter

Question
Can you tell me where it is located in my 1999 Ford Expedition

Answer
Most did not come with this filter. Look under the right side of the dash, the bottom of the heater case is black and is either covered by a site shield, or you will find the bottom of the case has a recessed area where the filter WOULD go, with four screw holes that aren't used. I have yet to see an Expedition with this filter from the factory. I understand you can add this feature, but I don't know how much work is involved.