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Hello, just to be clear you are talking about the selector switch for the fan speed?
Ok I am going to look over some info and will get back to you asap.
At idle with the air off where are your rpms at?
If the rpms are to low at idle and you add another load such as heater/a/c system it can the car to stall. The first thing you need to check for is a dirty throttle plate. You will have to remove the air intake tube off the throttle body to visually inspect throttle plate. If its dirty clean it with a rag and solvent like brake clean or throttle body cleaner. Spray the cleaner on the rag and open throttle to clean the plate and bore as best you can. After doing this it should raise your idle and you can see if this resolved your issue. Let me know how it goes.
Any luck on your issue?
This is a procedure you can do yourself if you want to save some money.