That indicates that the servo is operational as well as at least part
of thw switches on the the control stick.   The other components are
the cruise control amplifier and a speed pickup, which is probably
located on the speedometer in that model.  Of these, it's usually the
cruise control amplifier which is kaput.   The Germans aren't very
good at electronics.    You can try removing the amp, opening it up,
and resoldering the solder joints.   I've seen it claimed that most
failures are the result of cold solder joints working loose.   But
with personal experience, I've never gotten lucky trying that
approach.

They should be available as rebuilt ones from parts places for about
$180.    Also, I think General Development Labs either rebuilds your
own or offers rebuilt exchanges.   Not sure where it's located on that
car.  On my 1980 300SD, it's mounted under the dash, in the parking
brake/regular brake pedal area.   It's an aluminum rectangular box,
maybe 6"X4"X1"

It does not stick to any speed.

Rebuilder:  http://gdl-online.com/begin.html