Are you sure it has a 5th gear? My C55 in manual mode, goes up to 4th then into D.
Are you switching to 'M' mode? Otherwise all you're doing is extending the gear range and once past 4th it just reverts to D.
Okay. I just went out in my 2004 - W203 C55 Wagon.
In 'M' (Manual) I have gears 1 >5 NB. I can only get '5' above (I think) about 50MPH. So if I sit in the car at idle with my foot on the brake and bump the shift or use the paddles it will not change up or down.
In 'S' (Sport) I have gears 1 > 4 then 'D' when I use the paddles or the shifter. If I just start it up and drive it is always in 'D'
So in 'S' it displays 'D' as a substitute for '5' and will only display gears 1 > 4