This is a vague memory from taking apart the ACC pushbutton unit on my 1987 300D, and your unit should be similar.
I think there are two light bulbs in the unit, and the roller wheel gets its light by a plastic optic rod from the left light bulb. So you really should see that the whole left side of the pushbutton unit is dimmer.
To replace the lights, you have to remove the woodgrain panel, and then there are some pretty small holes through which you can access the "grain of wheat" light bulbs. In some cases these holes may be covered with a small piece of tape.
1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles