Set the dimmer on highest position. Now check instrument panel for discoloration. Check if some parts are brighter than others. There are numerous small light bulbs in the rear and you should be able to notice if one of them is burned.
If it looks good then your cluster is probably just not bright enough for you. I'm not a big fan of the old Mercedes clusters either. I wish they all switched to the 2000 S-class clusters (similar to Lexus).