As typecastboy says, you need a wet vacuum to have any chance of getting the grime out of the seats; the spray on foam jobbies just redistribute the grime more evenly; the seat may look clean, whilst in reality it's just more uniformly dirty.
If there is a uniform layer of grime left behind, then any subsequent water spotting, even by the purest possible water, will leave a stain as the water redistributes the grime that's already in the seat covering.
The original 500 lounge seats are notorious for being about the worst thing possible when it comes to showing stains; after about 2010 Fiat made the pop seats available as a no-cost option in the lounge; they're a far better choice as the fabric is more durable, too.