Jesse, that brown nightmare has got to be a spilled can of Pepsi, or a big
ol' latte, or something else that just seeped into your foam.  Relentless,
repetitive cleaning unless you can pull off the upholstery and get straight to it.

Since I have many pets, I own a Hoover Steam Vac and it does a good job of
pulling up the crud.  You can flush as much liquid through it as you need to
really clean up.  If you have a wet/dry shop vac, you can achieve similar results
using a spray bottle or watering can to apply the cleaner, then vac it dry.
I am very lucky that my van interior is the same color as my dogs.  :-)

I've got a small, similar recurring brown spot on my tan interior, in the
back on the bed cushion.  Maybe the PO fell asleep with drink in hand...

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL