I have a 2007 Dakota, Quad Cab, V-8. For some time now, I've noticed a dark, oily-looking stain on my carpets along the bottom of the doors. It looks as though it has seeped in from the edge of the body at the bottom of the doors. See photo. I bought the truck used, about 6 years ago, so I don't know if the previous owner did something. I've tried cleaning it with slightly soapy water, but that didn't even touch it. Solvent (varsol) seems to have some effect on it. It's all along the bottom of all four doors. Any ideas what is causing it?

 

Maybe one of those kids in the back spilled a 44 oz. coca cola? I say this because it happened to me before, and looked similar.