Vents are completely functional
The vents are functional. I went to a junk yard and walked around for over an hour trying to find the smallest vents I could find. I ended up getting them out of a 1996 camaro. Actually, I had to get them from two different Camaros as there is a big one and small one on each car above the instrument cluster. I did not damage or alter anything on my Trailblazer to do this install. I cut a peice of plexiglass with 2 holes the size of the vents and mounted 2 peices of old flexible vacuum cleaner hose on it with bondo body filler. I then simply put a screw in the middle and mounted in the hole where the factory vents butted up against it. It was quick and simple to do. I left a small gap from the hose to the new vents to allow air flow to keep heat from building up from the 8.5" LCD touchscreen. The 8.5 screen is as big as will neatly fit into the recessed original vent housing hole. Putting a bigger screen is possible if you eliminate the vents altogether. I recommend not going any bigger because it will block your view. This one is about two inches above the top contour of the dash.