This is all from memory: we traded in the 406 last weekend.

First lift the bonnet and remove the plastic grill/trim just under the
windscreen on the passenger side of the car. This will reveal the air
intake. Look down into it to see if the flap is open or closed. The
servo motor will probably still work enough to open it if it is closed.
Don't put the grill back just yet.

Remove the glove box. It's held in place by two white nylon pegs that
act as the hinges and simply slide out. Once you have that out of the
way, you should be able to see a small inspection plate (~10 cm square)
that is held in place by one (I think) Torx screw (can't remember what
size). Remove the plate and look up into the dark void above using a
torch to light your way. If you feel around, the main object up there is
the motor. If you are not sure that you have identified it correctly,
turn the re-circulation on and off a few times. You should be able to
feel the motor vibrating slightly.

To disconnect the motor, feel around for the pair of wires and
disconnect them: it's a simple pull connection. BUT FIRST, check that
you really have left the re-circulation flap open. You can now put the
grill back under the bonnet.

I never tried to get the motor out, so cannot help you there.

Once the motor is disconnected, the light on the re-circulation botton
on the dashboard will still work. That's good when you come to sell the
car: no awkward questions from potential buyers.

Hope this helps,

Raymond

I found the most essential piece of equipment was an agile son-in-law, as
you are working under the glovebox in front of the passenger, at times on
your back.

You should find that the flap still opens and closes (closed when the light
is on, open when it is off) but the motor continues running and clicking
when the flap reaches its limit of travel.  You can also see the flap by
removing the grid outside under the passenger side wiper and the filter
below it.

Remove the glove box by taking out the plastic hinge pins.   Behind it there
is a black panel, secured if I remember correctly by one screw.   Remove the
panel.   The motor is through the hole and round to the right, put your hand
through and you will find it.   Operate the recirc button and you will feel
the motor running.    If you want to remove the motor it is held by two
pozidriv type screws, in opposite corners.  Otherwise follow the wires and
find the plug, it is tough but will pull out.   I tried to get a replacement
motor (also apparently called a servo) from a scrappy but could only get the
later type which will not do.   This link will take you to pictures of both
the original and later type motors.
http://uk.geocities.com/keith.willcocks@btopenworld.com/Recircpics.html

Hope this helps