F-150 heater/AC blower motor problem

The dash control switch for the blower speed no longer adjusts the blower motor speed. I've isolated it to the resistor board, but can't for the life of me find where the blower resistor board is located. Does anyone know where the resistor board is under the dash or in the engine compartment?

thanks

It should be somewhere inside by the blower motor. Inside the air duct box. Try removing the motor and looking inside for it.