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Hello and thank you for your question,Can you tell me the year of your Mercedes 300E please?
The most common caused for the blower on your 300E to not operate is in most cases caused by a failing Blower Speed Control Unit with the blower being second most. Before you consider replacing any part you will need to to a bit of testing. I have enclosed the test steps in the link below .https://dl.dropboxusercontent.com/u/104923897/Blower%20motor%20Jul%2021.pdfSee the wiring diagram as well for reference.https://dl.dropboxusercontent.com/u/104923897/Blower%20wiring%20Jul%2021.pdf
Your very welcome and thank you as well, that is the most common part to fail for sure.