Vent fan switch

[55AS]

I cannot locate the switch to activate this vent fan in my 54 FC. I thought perhaps the pull chain that opens the exterior vent would activate it but it is a separate device. Fan wires enter the wall headed upward but there is no obvious switch in the cabinet above or on the ceiling. What am I missing?

[xrvr]

Is there an outside flap that must be open for the switch to operate? Jim

[55AS]

Found the switch (activated by opening flap) and it works but fan motor is dead. May have to buy a whole new fan if I cannot find a replacement motor. Any idea what hp, rpm might be? There is no nameplate.

[55AS]

I ended up replacing the entire fan with this:
Amazon.com: Broan Replacement Range Hood Fan Motor and Fan - 2 Speed # 97012248, 1.1 amps, 120 volts: Home Improvement
Had to modify the bracket to fit (cut, bend, drill) and rewire but now works like a champ.