The fact that the car does not warm up and the heater doesn't produce heat is usually a sign that either the thermostat is stuck open or has been removed. If it has been removed it could have been either because it was faulty and the PO took it out because he was too lazy to buy a new one or there was some other problem like overheating and removing it was a "free repair". Either way, you won't know until you at least try to fix it right by replacing the thermostat first and see what happens. As others have said here, there is no reason to use cardboard on a Mercedes.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz