This is quite common on the '98 C series. I didn't know it affected the E series as well but they probably got the controls from the same place. There are 2 different problems with the displays:

The problem that usually occurs first (at about 5 years of age) is some of the segments start to disappear. This sounds like what you are describing.

The other problem that happens at about 10 years is the whole display starts to darken (all the segments getting dark) at one side or the bottom. *edit* This second problem affects the RADIO display not the climate control. *edit*

Assuming you have problem #1, I can tell you what I do about it and what others have said, and you can search.

What I do is just tap hard on the glass with my finger. One good whack cures it for a while, a week or so.

Some people posted that there was a loose connector inside that you could fix but I am skeptical based on my experience with LCD displays. However I have not been motivated enough to take it our and try. And now I am getting problem # 2 also.... (on the radio)

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra