The cover is at the bottom of the windscreen on passenger side. The
cover of the pollen/cabin filter forms parts of the windscreen seal.

"Giulio" wrote...

if you mean 'Focus' then see Alan's reply.

if you maen 'Fusion' then (at least on the right hand drive model) the
pollen filter lives in the side of the internal heater assy, to the right of
the passenger footwell.  Access is via a plastic cover with (IIRC) a couple
of screws and a little encouragement.  Be gentle, the locating flanges to
the rear of the cover will snap at the first opportunity and cackle at you
in revenge...

hth.