I know it sounds silly but what you really do ? Cause for me personally when i turn on defog itself as usual the car basically blow cold ac air on windows and most of the times it just doesnt work that good. And i tested it on multiple cars. I tried just setting the car the blow outside air with no ac and that kinda works better. And recently i actually turned on heater to blow on windows and specially with ac on at the same time with heater and it works much better