Kickdown switch?
 

I know the auto trannies on the w123's use vacuum control and a bowden cable for shift control, is there also a version that uses an electric kickdown? Reason I ask is that my 80 has no bowden cable, or provision for one, and when cleaning the driver floor yesterday I pulled the carpet and found a round black button type switch hiding under the accelerator on the firewall, with a flat connector on it. I can mash on the pedal and it never does kick down until I pull the shifter down a notch, been that way since I've had the car.

All those transmissions also have electric kickdowns, even the ones with Bowden cables. Yours must have a broken switch or connection.

I don't know if it's electrical or not, when you press down, you should here a "snick", even with engine off.

cdplayer
 

My first Mercedes was a W123 (300CD)
The kick down feature didn't work either. One day while the car was raised I found a loose wire tucked almost out of sight near the rear of the tranny. Once I found where to plug it in, I realized I had kickdown!!
Could this be your problem too?

I'll have to check my switch and wires, I know the connector on the trans is broken open, the backshell clips have snapped, seems to be a common problem as every one I see in the wrecking yard is the same way. The pins still seemed to be holding in ok on the trans though last time I looked, I'll have to look in my body manuals and see if there is a wiring schematic that would tell me what pins on the trans to ring out, or is it just that single wire that goes to the solenoid on the side?