Ive had this problem too, (theres another thread floating around on here with the same thing.) In that thread someone said to run the ac on recirculation for 2 weeks everytime you drive. Well i did that and after about 4 days the smell went away. i've found that if i run the A/C with fresh air coming in the mildew smell starts to kick in (the smell is very mild in comparison to what it used to be) but i just use the AC on recirculation now and i never smell it anymore.

i also make sure to turn off the AC before i turn off my car, the car then switches to fresh air mode with the AC off, not sure if that helps with evaporation at all but i do it anyway.

 

I have changed my cabin filter many times, it is not the source of the mildew that is present. It is more in the evaporator core. I think our cars don't extract condensation well.

I even traced where the drain hose goes through, which is under the passenger carpet to see if there is a leak or it is not draining adequately. It is fine and it is draining.

Sucks cuz this is a nice car, everything about it is nice. but my mom's 2006 rx400h doesn't smell one bit.

I did use a a/c evaporator cleaner that seems to work, but the smell eventually comes back in a couple weeks.

What I would do is
Remove the cabin filter (behind the glove box)
and put the fan on full blast and spray the a/c evaporator cleaner for about 5-10 minutes. Eventually the mildew smell goes away. But does come back,

I didn't know have a 36k+ car required this kind of nuisance.