I have a 83 300CD which is probably similar to your 300D.
I have had the same situation and all it took was to remove the wood trim around the climate control unit to give you access to a plastic plate behind it.
I removed the mounting screws of the plastic plate to get access to the bulbs.
Mine had a continuous lighting wire that supplied power to two lights for the climate control unit and one light mounted to a socket that had four hollow plastic tubes that ran to the four switches along the top of the center console. This one bulb lights all four switches by passing the light from the bulb through the plastic tubes. Replacing the bulb is cheap and easy however re-assembling everything is a little tricky because there is not alot of slack in the wiring with everything mounted. As for the bulb I just took the old bulb to a local auto parts place and they looked-up the specs and found a replacement for it.
I hope this helps.