Your actually doing more damage to your seats than good with a Magic Eraser. Thats a very abrasive cleaner and is eating away the seat vinyl as you rub. By removing the top layer that conatins the UV protectant, the spots you just "cleaned" will yellow over time.
a good seat vinyl cleaner (even a diluted bleach solution) is better for long life of your vinyl. Followed by plenty of sunlight and then a good rub down with some 303 solution will keep your vinyl looking new for years to come.