In your avatar pic your 4runner looks like a 99+ Limited... If that's the case, it should be equipped with a climate control system that is controlled by electric servos/motors.
On older 4Runners the climate controls are attached to mechanical levers that open and close the air mix vents and change the arrangement of the airflow from defroster to face to foot to face/foot mix, etc.
Since yours is controlled electronically, when you change the position of the temperature control, it sends an electronic signal to a servo motor that changes the position of the air mix flap. When you adjust it to the highest temp setting, that servo motor is trying to change the flap into a position where the ducts are being fed only by air forced through the heater by the blower.
I think the servo motor controlling that flap may be stuck or malfunctioning. That would explain the quiet blower, and no heat, and clicking. That's my thoughts on it.
EDIT... Just saw the title of the thread, "Problems w/2000 LIMITED" duh... guess I didn't have to judge by the avatar.
