I have had the center outlet out on my 98 ML320. There are 4 springy plastic protrusions molded into the outlet housing that index into the dash. The bottom two are exactly centered under the two outer nozzles. Pry at the bottom in that area with a 1" putty knife or such. The bottom will release and you can pull the whole thing out by letting the housing rotate slightly upward as you pull direclty out on the asembly.

The housing has a front and rear section and may separate at the center, where the damper blades pivot. No biggy. Just put it all together and then push the entire assembly straight into the dash. It you have it aligned properly, it will snap right in. If you miss, you must pull it back out and start over.

Good luck,

Jim in PHoenix