M.R.
Where were you storing the car seat? I recently got an extra high chair out of our attic and it also had a weird smell on the fabric cover. I washed it over and over again until I really inspected the whole chair. I guess the heat of the attic slightly melted the plastic of the chair onto the fabric. If this is the case you are probably safer replacing the seat. If it is just the cover maybe consider making or purchasing a new cover from the manufacturer.
Congrats on your upcoming arrival.