Downshift is controlled by an electrical switch located under the gas pedal. You'll probably have to lift the carpet to get to it. Sometime, debris builds up around the switch and prevents downshifting. Sometimes the carpet bunches up. Sometime the switch gets funky and has to be cleaned or replaced. Power for this switch comes from the Klima relay, which may not sound logical, but that's where it comes from. So a bad Klima will cause the downshift circuit to fail. When the switch engages, power goes to a solenoid on the back of the transmission. I've had the harness connector fail at the solenoid. It's a weird terminal, and needs to be sourced from Mercedes. Where I would start is to lift the carpet to expose the switch. I'd temporarily clip a momentary contact switch in parallel with the downshift switch. Then I'd take it on the road and test. If the temporary switch does the trick, replace the downshift switch. Otherwise, trace the power and replace the Klima, solenoid, or connector as appropriate.