If the switch your talking about for the 12v fan in foil tube in tank bays, for gas furnace operation, its located on water closet panel marked fan.
The fan will only run when the furnace does and their are fuses two or three taped up behind the wood panel behind the return air vent under fridge and one of the fuses is for the fan.
If you have a OASIS System it has a heat sensor in water bay that
looks like this.