More info from Sixto.....

sixto
Senior Member

Registered: Jun 2000
Location: SF Bay Area
Posts: 3999
The flat spot beside the button is the LED. It looks like pin 2 filled in. Key on, engine off, hold down the button for two seconds then count blinks. Hold down the button for two seconds then count the next set of blinks, etc. Hold down the button for 10(?) seconds to clear each code after reading it.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD