What an awesome forum and helpful folks.

J. Hidalgo got me looking into removing the dash (yikes!) which led me to this post:

http://www.peachparts.com/shopforum/showthread.php?threadid=35373&highlight=dash+removal

which describes a problem with high mileage 124's and corrupted vaccum actuators (i.e., with holes in the rubber diaphrams) that cause the ACC system not to work. He's made available lots of pictures at

http://www.meimann.com/images/mercedes/W124_dash/

I'm off to look into Earl's suggestion that the problem is the temperature contol switch. I've actually been inside this part of the dash recently as I've replaced a non-working aftermarket radio.

Guess which one I'd rather be the problem?

I need to ask again: the MityVac shows that the vacuum reservoir (now out of the car and on my bench) will not hold a vacuum. Though I'm sure it's a part that fails rarely, doesn't this suggest that it's a likely candidate for replacement?

Thanks.

Sean