From what you describe Lance, it sounds almost as if the dye in the vinyl has been removed to a degree, in that area. This is not normally attributable to the products/cleaners you have used on them, so don't blame yourself. It could be due to a poorly impregnated dye from the vinyl maker, or Volvo. How old is the car?
I know Ford often sprays some of their interior vinyls for door coverings , seatbacks etc. with a color (they order vinyl in lots and they aren't all color matched for their various interior shades, so they spray them at the factory) that can sometimes come off with a strong alkaline cleaner (Simple Green can even do it ) . I have even witnessed dye removal with the use of some leather cleaners on poorly prepped/bonded dyes. Kind of scary.
Of course this is just my guess for you. If it's true, you can buy vinyl dye (black shouldn't pose too much of a problem ) in OEM matched shades and respray that area. I have sprayed entire vinly and leather interiors with SEM brand products, and it's permanant. Black leather shoe dye may even hide it , but not sure how well the blend might be. Whatever you would use, cleaning and prep would be key.
If nothing you're doing is working to alleviate the problem, it could very well be a lack of pigment there. Good luck, hope this helps.
