Your heater won't thermosyphon beceause the tank heater is mounted too high. The cold water should fall into the heater, get heated, then rise out of the heater. Mine was mounted on the wheel well - passenger side, battery is left front on my model - water fell out of the second-from-the-rear frost plug then rose into the return line for the heater core.

Where are your tank heater hoses connected? I may have an idea.

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daBenz - 1970 220D