try this url....the other isn't clicking over for whatever reason...
you have to start at the top opening of the vent....I pushed down on the vent and snuck my finger in the top as it was giving....then sart working it down and out until it pops.....the vent is snaped into place by 3 sets of clips....work your way down and be careful because it requires A LOT of force....the vent will bend and give, just don't get frustrated and scratch the paint....there really is no great way of explaining it but these pics might help...
http://www.ppbb.com/boards/ppbbphp/showflat.php?Cat=&Board=ARCHIVES&Number=1025818&Forum=ARCHIVES&Words=987%20vent%20&Match=Entire%20Phrase&Searchpage=0&Limit=25&Old=allposts&Main=1025818&Search=true#Post1025818