Turning the fan to "off" on the left most lever, doesn't always turn the fan completely off. The fan speed siwtch is attached to the back of the heater control panel and operates off a linkage that operates the dampers in front of the heater core. When you slide that lever up to the "AIR" position the fan should stop, but if you inkage is worn, or bent the switch may never switch the fan off an the fan will continue to run in slow speed.
AC equipped cars fans will continue to run even with the fan switch across the top of the heater control in the OFF position. It will not shut completely off until the tmeprature cntro switch is placed in the OFF position as well. These cars have a separate "lo Blo" switch attached to the bottom left rear of the heater control that keeps the fan running in slow until the temp control switch is turned off. If you put the temp control switch in off, the fan won't work in any speed, as power is completely disconnected to the fan switch.
