There is a dimmer knob on the 300e. It is the same knob that is used to reset the odometer. Instead of pushing it in, just turn it to adjust the brightness.
You may need to replace the bulbs. I had to do this on my 1988 300e. The wife complained that it was not bright enough for her. The adjustment knob was turned on full.
Pulled the instrument panel and replaced all of the bulbs in it, about 4 or 5 if I remember correctly. Now all is fine.
Hope this helps.