According to the FSM for the 107 chassis for '78-'79, the ground wire for the aux heater pump is the brown wire. This means that the red wire(from plug to pump) or the black (<yeah, I know it goes against conventions, but thats per FSM>, from the plug to the switch) is the positive wire.

__________________
". . .back before accountants designed cars"

-Current Stable-
'78 MB 450SL-C 107.024.12.020783 #3840 <Kayleen>
'85 FORD F250 6.9L Diesel <Allison>
'98 Lexus ES300 <Rachel>
Long Gone...
'74 Chevy G10...........................'99 GMC Yukon 4X4
'83 Chevy Suburban 6.2 diesel .....'99 SAAB 9-5
'90 Mitsubishi Eclipse GS............. '01 Chevy Tahoe
'98 Nissan Altima .......................'02 MB ML320
'88 Chevy Suburban V2500 4X4 6.2 diesel