This is really easy..

The glovebox is a self contained item including door.

You'll find 5 hex screws to fix it. If you open the door, there are
three screws on the top of the opening. One left, one right, and one
in the middle close to the lock.

You'll find the third screw if you trace the outline of the glovebox -
which at the same time is the edge of the dashboard there - at the
door side. Down somewhere there is a gap where the glovebox ends and
the screw is there.. easily accessible from the footwell.

The 5th screw is really close to the central console.. about the same
height/level as the 4th screw.

Open them all, the glovebox comes out easily.. just be careful about
the wire connected to the glovebox interior lighting. You will have to
unplug that once you lowered the glovebox a bit.

Installing the thing is similarly easy, just fiddle a bit until the
box sits nicely in place.. no noteworthy forces needed.. unless you
have a hands free kit, and a whole bunch of cables for your alpine
navi and stuff behind it *cough*

Regards

Wolfgang
(A6 Avant '99)