It probably IS a stuck open thermostat, which is the way it is supposed to fail. Replacing your failed thermostat will increase the heat from your heater, and make your engine run at the proper temperature too. Make sure you have the rubber seal around the t-stat. The old one can be as hard as a brick.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.