Remove the right lower panel and knee bolster (removing r. lower dash screw helps get the knee bolster out/back in).
On the right side of the heater box is an electric motor. Remove that motor and look at the plastic driven part that goes around the metal drive part of the motor, it's probably cracked (the cause of the problem). If so, try using a small hose clamp, plastic tie strip or whatever to go around that plastic part to keep it from spreading by the motors movement. I've had good luck doing that vs the very costly alternative, a new heater box (big $$$).