My wife and I did not like the A/C vents in my Miatas. They would not stay positioned and much to my wife’s chagrin could not be turned off individually. She did not like the A/C blowing on her all the time.
I found this project to replace the vents with louvered vents from a Mitsubishi Eclipse. http://www.mx5atlanta.com/forums/showthread.php?13490-3rd-Gen-Mitsubishi-Eclipse-Dash-Vents-in-an-NB . This thread shows in detail how to prepare the 3rd generation Mitsubishi Eclipse (2000-2006) vents for installation. I used the instructions and found them accurate.
However, the removal method for removing the Miata vents is an issue. I read several ideas on vent removal on http://miata.net/ and other sites. Most warned of breaking the tabs that hold the vents in, and or, marring the dash. I was concerned about removing mine until I found a thread that suggested using plastic cards like an old credit card to slip in beside the vent and push the tabs out of the way.
I had some old plastic ID cards that were made of the same plastic as credit cards. I cut a card in half and tried removing a vent. It was hard at first since the card will bend if not carefully held. I found that pushing it in at an angle just above where I thought the tab was located worked well. As it touched the tab, I would wiggle the card and eventually, the tab would move and the card will slip in between it and the vent. Then I did the same to the other side of the vent. When the second tab was pushed back by the card the vent would literally pop out.
I have used the plastic card method many times now and have not broken a tab or marred any dashes. I have also used the plastic card method to remove the Mitsubishi vents to show a friend how they are removed and installed.
Another concern was some threads stated the vents would not rotate. I used some white lithium grease very lightly on the section where the vent meets the dash and where the tab engages the ring on the vent. All of my vents will rotate. On a rare occasion one will try to pop out while turning it. The ring is not very deep and could be cut deeper to prevent this, but I find that a slight push re-seats the vent and it has not been an issue. It rarely happens anyway.
All of my Miatas now have Mitsubishi Eclipse vents. These vents look like factory installed vents. They are awesome! My wife loves the ability to turn her side off.
This is the most useful mod I have made to any of my Miatas to date.