If that doesn't work get them steam cleaned, check craigslist for upholstery/car cleaning. My dad is a carpet cleaner so luckily I can use his 10 gallon portable cleaner whenever I want and when I detail cars. He said he used to do cars all the time but doesn't feel the need to crawl all around in cars due to his back.
Somebody around you has to do it, even with how clean your interior looks. You would be amazed at how much brown sludge would be drained out of the machine if you got it cleaned properly. The last customer I detailed a car for almost puked when I showed them the bucket of shit I sucked out of their seats and carpets. And it was a 2008 with tan interior and the interior looked clean so I felt like I was wasting my time at first.
The brown water in the bucket definitely did not look like it came from the car and it proved the process worthy.
this is what I mean