Have no consistant heat inside my 1978 450sl.
When the car is started, the defrost blows heat, but if any other mode is selected (hi low bilevel) cold air begins to blow. Then, if you reselect defrost, it too blows cold. No matter what you select, you get cold air, until the engine is turned off and then restarted with defrost selected. With defrost selected at start up, hot air blows consistantly until you attempt to select another setting.