Definitely try OxyClean!
Just before DS2 was born, I decided to wash the carseat cover. DH took it out of the washer and tossed it in a basket, all wet. When I found it, it was
covered in mildew and mold. *sob*
I took a small water cooler, filled it with HOT water and a bunch of OxyClean, and put the cover in to soak. Every few hours, I'd take it out and scrub it, then put it back in to soak more.
It was an all day chore, but it all came out!!

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