My windshield wiper cleaning fuid only works when it's raining out. Sounds silly, but when it's dry and dusty, need that liquid. Any suggestions?

You need to provde year and model of your Benz - it helps those who read your post out alot. Some newer models have a rain sensor.....

You can try clicking on his profile and sometimes get the info you need. Says he has an 87 "SDL Turbo", I assume a 300 SDL.

No real idea why this would be happening, but I'd try having a helper run the washer system with the hood open and listen to or feel the washer pump to see if it's running or not.
If it is, there possibly is a leak in one of the hoses. If it doesn't run, then use a test light and also with the washers activated and the electrical connector removed, see if there is power to the pump.

Also, if you are in the habit of turning on your headlights when it rains, you may also have a situation where the headlight washer pump and windshield washer pumps are plugged in incorrectly.
To test this theory, try turning on your headlights and then see if the windshield washer works. If so, you need to just disconnect the wiring from the 2 pumps and plug them into the opposite ones.

Gilly