I have a 1987 300D so my model should be similar. Once you take off the wood-grained cover to the climate controls, there are two tiny holes (like 1/4 inch or smaller) for access to very small light bulbs. In my situation they were both burned out, so I had no lights at all. They shine through a clear plastic that is designed to diffuse the light to all the buttons.

In your case, I expect the left bulb is burned out, so you don't have light behind the thumbwheel.

There is probably an easy way to get these bulbs out - but I don't know what special tool could do it. I was going to disassemble the unit anyway to resolder internal connections, so I changed out the bulbs while it was apart. This seems like an excessive amount of work just to change the bulbs.

Hopefully someone can tell you how to change them easily. I can at least tell you where they are.

Ken300D