The plastic ring with the chrome trim comes out from the front, the actual vent comes out from the rear. Once you remove the hose from the back, you just kinda gotta push in, towards the rear of the car with one hand, and hold the ring steady by pushing towards the engine, and twist at the same time. Be careful not to break the tabs, etc.

Center vents are easily reached with the glovebox out. Not sure about the driver side vent as I've never replaced one on that side. I'd think removing the cluster would be the way to go...

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)