I installed a center dash vent (CDV) on my '87 535is over the weekend. Wait a minute...I should say that I made what BMW had already installed at the factory functional by adding two parts. A buddy of mine from NY found the "new" vent plate and the roller knob out of an '84 e28 in the salvage yard and mailed it to me. What a nice guy! Anyway, the rest of the hardware is already in place.
Others on the forum have written about how BMW NA disabled the CDV for the North American market because some owners were confused about the HVAC controls...go figure. I'm not sure when BMW started disabling these vents for the NA market.
Anyway, it was incredibly easy to install...simply take off the old vent plate (2 phillips head screws), unclamp the vent lever (actuator) that BMW retained with a metal clamp, pop-in the roller knob (make sure it is facing the correct way), snap the vent lever (actuator) into to the hole in the knob, install the new vent plate (with the hole for the knob), reinstall 2 screws and voila....you're in business. All in all, a cheap, worthwhile addition to the car.
87 535is 5-speed 152k
(now with top CDV enabled)