I don't know about all MB's but on my 84 500SEL the trunk and fuel tank is heated from the air flow through the car. I haven't actually checked the heat in the trunk but I did once check the temperature in the trunk during the summer. Inside the car the temp was 78 degrees, inside the trunk it was 88 and outside was 94. So I guess the same could be true for heating in the winter and the fuel tank is in the trunk. On a long drive, the fuel could be quite a bit wormer than the outside temperature.

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84 500 SEL (307,xxx miles)