The heater in my '96 makes a pretty noticeable difference in glow time and starting ease. It works better in an enclosed or semi-enclosed environment, a place where the vehicle is shielded from winds. One thing you can do to verify if it's working or not would be to plug it in for at least a couple hours, then pop the hood and feel the engine. It'll feel warm to the touch if you've got a working heater.

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- Greg -
1973 220D, The Prodigal Benz
1974 240D