[JC455]

The fresh air inlet is on the RH side of the cowl (passenger side), under the plastic screen. There is a vacuum actuated air door that opens to allow fresh air into the plenum.
The pass. side kick panel has an add-on bubble-shaped cover that goes over the vacuum door from the plenum/cowl area. You can remove the bubble cover and test it w/ vacuum, or watch it for movement.
Did you check the squirrel cage on the blower motor? They are hard to get to, due to being under the pass. fender/behind the inner wheel house, but the factory put an embossed circle on the inner wheel house to cut out for servicing the motor.