What year car? If you are talking about the BLUE switch(2 pin connector) on the front of head, then that is probably the problem. On 1988 & later cars that sensor is a varible resistor & must send the correct info to the A/C pushbutton or the fan will run! Unplugging or jumping this switch makes the fan run!
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR