YOu have to have the vents set to recirculate, and then spray your sauce. Give it a really good spray, and let it sit.
Repeat as necessary.
Often having the AC ON will effect the recirculation mode.
If you spray with normal venting from outside air, all you are doing is spraying the car interior, not the innards of the system.
Also, make sure your drain is open. A straightened out coat hanger from the bottom of the car works.