Hi I too have the same issue I have A C230 2003 and I have seen it getting worst now it is hard to read the dash display. Where you able to find out how to fix the issue?
I found this information you may want to try this if it helps Im going to try it this weekend:
Replacing the DASH DISPLAY LIGHTS
If the display darkens (background *and* numerals), it could be the
backlight(s) for the LCD display. LCDs are variable filters which must be
illuminated from behind to actually display anything. The lamps used are
typically like the 1.5 watt bulb seen at the bottom of this page (Osram bulb
with beige plastic base $1.58 each, as what my CLK uses):
If they are the culprit, they are accessed from the backside of the
instrument panel. I meant to take and post some pics and details of my CLK's
instrument cluster when I had it out, but have not had time (sorry).
Basically, if the steering wheel can be moved down or extended, do so... the
extra clearance makes it easier to work on the panel. Next, two hooked
removal tools (available from Mercedes or can be made from stiff wire) are
inserted between the dash padding and the cluster, one each at the ten
o'clock and two o'clock positions, they are then rotated, and the instrument
cluster can be pulled out:
You may have two electrical cables on the back... one on each side. When the
instrument cluster is out enough to gain access, slip the (gray?) retainer
down and pivot out the connector, do the same for the other side and the
dash will come right out. You'll see a bunch of lamps on the back side.
Replace the ones that are behind the problem display. Of course it may just
be a loose connector (less likely, IMO), but the cluster would also have had
to been removed to reseat them.
Reassembly is a snap. Put the cluster back near the opening, re-insert the
cables and pop their retainers back up, then push the cluster back into the
opening... voila... finis!
Sounds tough, but the whole shebang can be done in well under an hour the
first time through it... if you ever need to do it again for other lamps,
you'll be able to do it in fifteen minutes or less (hey, you're a pro now!)
THIS MAY *NOT* BE THE ROOT OF YOUR PROBLEM!!!