Would like to know which of the two wires is the positive on the aux water pump for a '79 300sd. Thanks

to test a pump, polarity is immaterial (if the direction of coolant flow is not important - which should not be for a short test). Be careful not to short out the supply leads as the contacts on the pump are very close together.

The aux pump should be running when the car is idling and the interior set to a temp that requires the heater to operate - being winter up north might make the last statement unnecessary ...

AFAIK the aux pump ensures adequate coolant (heat ?) flow through the heatre core at idle when the main pump is running slower ...

if the control unit is bust, the aux pump will not operate. A faulty aux pump will cause the control unit to pop .....

I'm not sure what the current flow should be, but it should not be more than an amp or two. put a fuse in to be safe.

L