Is the water draining out from under the car when you use the A/C? If not then you likely have blocked evaporator drains. You can buy commercial A/C system disinfectant that will kill the bacteria growing inside. I'm not sure if you can remove the cowl and blower on the W210 and clean the evaporator from the front. Otherwise it is a lot of time to remove the dashboard and interior from inside.

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1989 300CE