If you look inside the vents with a flaslight, you will see a couple of "tabs", 2 on top and 2 on the bottom. Starting with one side, insert a small screw driver and, carefully, pull down one of the top ones and pull up on bottom tab from the same side. At the same time, pull that side of the vent out. Do the same with the other side. It will come out.
I hope I am clear enough...Good luck!

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J.H.
'86 300E