With the A/C on, the fans should come on as well. What I see most often is the resistor and the wire burned to a crisp from heat. The resistor and the wire that burns are located behind the drivers side headlamp assy. They are hard to get to, but I'd bet money that are holding back the fans.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS