The switch typically wears out its contacts and/or spring after time. It looks like this:

[img]https://www.***************/secure/PartImages/0065453224.jpg[/img]

On the W126, the steering wheel needs to be removed to replace it. Not too bad of a job provided you have the muscle to untorque the nut. I'm not sure about the W123 as far as the requirement to remove the wheel.

The plug fits a socket behind the instrument cluster.

The climate control problems can certainly be the vacuum pods. Usually, after 20 years, one or more than one of them is NG. Right now, three of them are NG on the SD. I'm awaiting the diaphragm replacements to tackle the job, finally.