I have had the problem and it's because of mildew etc when the air con condensates. I used the stuff from Halfrauds which is in a can and sprays upwards, all over the back of your lovely seats
It smells industrially horrendous but the smell goes aftter 3 days and now I don't have any smells. I didn't think it worked at first but now all is nice!
Oh yeah, it cost £11.99 (eeeek!) and it's called Comma Air Conditioning Cleaner.