I think one of the enzyme products for organic pet stains/odors could work on this? Nature's Miracle is a common one at the Pet Supply store. Pour it on the area and allow it to work overnight. The enzymes can actually EAT the organic matter that causes the odors.
typically, to clean the carpets of my car, i just use a rug doctor and it seems to get rid of any smells that may exist and give the mats a once over. give it a try.
I agree with gardner man on the rug dr. thing. whenever i rent one, i'm sure to get the extension for upholstery with it ($3 extra). i'll go use the vacuum at the carwash and then hit the cars with the dr...you can see it lift any stains (coke, koolaid, etc) right out of the carpet/seats. i find the zep high traffic concentrate cleaner ($10 at home depot) alot better than the stuff they recommend for the machine, though...good luck with that