I have a 1988 f150. I was wondering if there was a cabin air filter because under my glove box but right above where your feet set there is a square hole in the cab. it seems to me that it has something to do with the heater and fresh air switch on the heater control. I would say it somewhere around a 8" x8" hole.

 

Lol, no.

 

Can't comment on a 1988, probably no way.
Later years, my 2012 FX-4 & my 2014 4x4 Lariat have Filters in each seat for the Heated-Cooled seats.
I believe in some higher levels in previous years there were Cabin Filters. I don't know when they started or when the droped the option.
I may be incorrect on the F-150's.
My 2009 Honda Accord had the Cabin Filter.

 

what do you think it is then

 

Be nice Sean...........Heater Core Dude! Yeah I know, come on Unit....let the dude learn. Oh well. I was a noob once. Probably the easiest heater in the history automobiles to change.

 

its not the heater core that's behind the glove box. im not dumb. its even below that

 

Is it an outlet for blowing air to the floor ?

 

I apologize sir! Thought was what you were referring to. I wasn't trying to call you dumb!