But a couple of them aren't there and are probably inside the vent.Looks like the ears are on the fin and just needs put back in place. Try moving it in place with a needle nose pliers.
You can paint the vents from the Olds. I have found several and painted them and the mechanism works just fine. You just have to take Your time when painting..Hang in there @Gael. You're getting some very good advice. There is a possibility that an aftermarket replacement may be available but it's still in the works so only time will tell. Although it's possible your vent could be repaired, it's dicey IMHO but might be worth a try. I had one repaired once, but it fell apart when I tried to reinstall it.
You could check with Cleveland Power and Performance:
As mentioned, Dicktator has a very good DIY guide for removing and replacing them. Look for "Dash Vent Removal":
FWIW, I had two vents that basically had no fins and two that were original. Bought four NOS from @cadiman and rather than risk damaging the new ones I pulled my dash to replace them all. Since I installed a new stereo a while back it wasn't as daunting as it was initially. If you take your time and follow all of the steps in the dash removal DIY in Dicktator's linked site, it's not that hard. Since the original vents are fragile as eggs, harder to find than hen's teeth and aren't cheap when they show up, I didn't want to risk damaging the new ones.
Also as mentioned, 1996 to 1999 Olds Aurora center passenger vents are the same style. I've seen blue and tan, but never a black one. I guess they could be painted, but I'd think that would really gum up the "mechanics". Doesn't seem likely you'll find many of those in Belgium anyway.
If you do decide to pull your dash and you're lucky (like I was), your dash may have already been removed at some point. If so, it's likely that the biggest obstacle, removing the passenger airbag, may not be too difficult. It's out of Dicktator's DIY guide sequence, but try removing the glove compartment first and then (after following the guidelines about pulling the airbag fuses, etc.) removing the two lower bolts that hold the airbag in. I did that and it turned out the the two top bolts - the most difficult ones to get to - had already been removed. As soon as I took out the two lower airbag bolts it was obviously loose. That allowed me to easily slip the airbag out of the dash without having to pry it out first. A huge time-saver.
Another alternative that I've seen others do, and depending on the condition of your other vents, is to pull all of your vents, remove all of the fins and put them back in. Although you can only direct the air left and right, they don't look that bad. If you go that route and have a couple of "good" vents, you probably could sell/trade them for someone's broken vents. Win-win. Of course if your other vents or in good shape it would be sacrilege to ruin them.
Just a few thoughts. In any case, best of luck and hope your hunt goes well!
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Gael, if You can remove the vent, then look to see if the tabs on the side of the vanes are still intact. if so, You can gently slip them back into their positions. If not, I have aligned the vanes looking straight forward and Super Glued them into place. You may not be able to direct air flow up and down, but the Vent will look complete and work Side to Side and forward.
I am pretty sure by now You have read the directions for removing the Vent. If you are gentle with it, it can be removed and replaced without removing the Dash. Just don't get in a hurry and DO NOT pull on the Vanes themselves, just the frame of the vent. Using a plastic pry tool, lift from the bottom and gently push the vent into the Dash. Rotate it clockwise and the Top should clear its' recess and allow You to pull it from the Dash slowly. Your Center vane is most likely lying in the void behind it. Those Little tips on the vane sides are so easily broken, not a good design by GM.. I have a few Vanes that I have had to Super Glue into place, no up and down movement but they look good and still blow front and You can swivel them side to side.