I'd bet the fuse on the low speed aux fan relay (stage I) is blown. Its a 15A fuse but I believe MB Doc or Arthur Dalton were recommending replacement with a 20A fuse per updated MB specifications. The relay is located just behind the fuse box (toward the windsheild) you'll have to remove about 6 phillips screws to get the cover off and the relay is blue in color.

About the A/C...what are the low/high side pressures, ambient temp, and vent temperature?