Circulating air switches of the old climatetronics

Hello, everybody.

after I could fix the stationary gas problem (OT-gage and idle-running stabilizer) as well as the start-up jerking (ABS sensors at the rear), I like my new V8 very well :-)

There's one more thing.
At the old climatetronics is a button "REC" on the left. It's supposed to turn on reciprocating. The LED above the button never goes out on me.
I can somehow determine whether a normal operation (with fresh air from outside) is guaranteed. Is there any flap or slider open?

Anybody know the problem?
If so, where should I start looking?

If possible, only the button "sticks" on.

Greetings and thanks
Jörg