The extender reacts on the door contact. I had a bad door contact on my 300CE ones and the right extender kept coming/going/coming/etc during the ride. I exchanged the extender and everything was fine again.
There are a couple of switches in the extender arm (4 I think) that determine how far the arm has to extend and retract. There is a relay (under the rear seat) that controls the (time-related) extended arm.

If you want me to, I can mail you some pictures of the extended arm system.

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)