Huge stain in back seat... help please.

I bought a 2013 Dodge Ram express maybe 4 months ago or so. I have noticed this hug stain that has appeared over time in the back seat. The more people have sat there the worse it has gotten...
The only thing i can come up with is that there is something within the cushion itself. And it is coming up into the upholstery when someone sits on it.
My question, does anyone here know how to remove the upholstery from the backseat so I can look at the padding and see whats there or try cleaning it? Thank you in advance!

is it cream colored?

If you've never done upholstery, in reality the best thing to is remove the seat, disassemble the bottom cushion from the firm and take it to an upholstery shop. Like you said, you probably bought a truck that someone spilled something in and the seat was cleaned (or even recovered - (the foam can be the most expensive part of upholstery work) - before you bought it.

You could look once you have the seat disassembled and see if you feel comfortable taking the cover off the seat, but if you are asking the question in the first place, the likelihood that you'd be able to put the fabric back on tight w/no wrinkles is poor i'd bet...

Cleaning the foam correctly and making sure it's completely dry may be problematic as well; it may in fact be easier to replace the foam, though it's probably not the cheapest route.

Another option may be to find a matching seat on Craigslist or in salvage.