Juan, thanks for your tip. Armed with that info, I checked the headrests and, as yours, they didn't retract. Seems we're on to something! Next, I looked for a disconnected vacuum line at the firewall. Nothing on the passenger side (checked gray and green vacuum lines), but when I followed the gray line over to the driver's side, past the oil filter and under the intake manifold -- ah hah! -- it was disconnected (thanks no doubt to some knuckle-dragger at Jiffy Lube).

Time for a test: I energized the ignition and hit the head rest button. Nothing. I started the engine, and hit the button again. Down they went! Next, the air-con controls: I cycled through the air-direct button and, for the first time in many months, I was greeted by a blast of cold air from the center vent. Yeah!

However, after a few minutes, and more cycling tests, the air stopped coming out of the center vent. As far as I could tell, everything else still worked (including the head rests). Darn.

I though the vacuum line had come back off. Nope, still connected. I tried driving it around the block, going from light- to full-throttle, with speeds up to 60 MPH. Nothing.

It's odd that the center air vent worked for two minutes, then quit again.

An hour later, I remembered that my trunk latch was also not working quite right -- apparently also a vacuum problem, and who knows, possibly related. That symptom: if you pressed the center console button to pop the trunk, you had to wait 10-15 seconds for the PSE pump (right rear quarter panel near the tail light cluster) to stop humming before you could close the trunk.

So anyway, I went back to the car to try the trunk. No change.

Then I started the car. Get this: instantly I had a blast of cold air from the center vent! What the ...? I cycled through the system over and over, all air vents worked as they were supposed to. At least for now.

I'll let you know how it goes, and if some Mercedes-Benz guru here can explain why this happened, I'd really like to know. Thanks.