It could be that the switch has failed or that the system is a little under-charged. I have topped off a few this year and the fans light up.
It would be OK to connect a set of wires to the two that are there and close the contact with a Manual switch. DO NOT HIT GROUND WITH THIS.
(I did this to my 190-16v to help with the heat)
You can even use this with the a/c off.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS