I bought a round black voltmeter with an ivory colored needle and numbers. Matches the other instruments in the car. I placed that where the antenna up/down switch was.

And due to the notorious electrical problems with these cars, I depend on that voltmeter for information. Just had a belt squealing/slipping problem. I saw that the voltage was 12 volts when squealing, then 13/14 volts when it stopped squealing! So I knew the culprit was the alternator belt.

You can also get voltmeters which plug into a cigarette lighter (on Mercedes they are called "cigar" lighters!)