I did a lot of testing on this a few months ago. But, this is only on my 85. Not sure if other years are affected.
I found that the aux. pump only runs when the defrost button is depressed or if the CCU wheel is all the way to max. heat (in the detent). In any other position, the aux. pump is off.
If you think about it.... It makes sense. These are the only 2 positions that demand maximum heat (coolant flow to the heater core).
1985 300D (SOLD)