IF the 5 and 3 are anything alike in this area, it will be located in the
center of the dashboard, just inside the cabin from the firewall.

In USA (left hand drive) cars, you go in through the passenger side foor
well. Pull the trim around the glove box, and the footwell. The filter is
located on a line just about directly below the radio. You may have to
disconnect a block of relays, or remove it from the mount and push it to the
side. Look for an access panel that is about 2 inches by about 8 inches. The
panel will have a twist lock to hold it on.

Makes a difference whether the car is left or r/h drive.
If it's r/h drive, the filter is in the drivers footwell. You need to remove
the panel that's in the footwell, under the steering wheel, complete with
it's ducting, after disconnecting the warning 'bong' speaker, which is
mounted on the panel..
The filter is to the left. More or less alongside the footpedals, and in
effect, behind the centre console. Quite awkward to remove and replace,
which is why on r/h drive models, the filter is in two parts.
Mike.