I think you are right(pretty sure). Ther is no switch on the trasmission. I do not know whay Mercedes made a simple function so complecated in this case (may be only for 98 E320).

I do not have the wiring diagram. Can you tel me the flow of the wire from the light assembly to the source. Where the valve body and the Transmission ECU. Thank you very much for your help. Unfurtunately I am not close to your location, other wise I would bring it to you for diag nose and repair.