Removing Melted Plastic from Car Seat

I am scared to even offer solutions but probably your best bet if it's really into the fibers to try heating it back up using the method you would use to remove candle wax from a tablecloth or furniture. Use a clothes iron on its low setting. Put a brown paper bag on top of the stain and press the iron on top of the bag (and the stain), leaving it for a moment, but not too long.

As the iron heats the bag, it should transfer some of that plastic from the seat to the bag. But, you will want to be patient, not to let the seat get to hot or you will have a new larger stain from the iron damaging the seat. If you can't see plastic transferring to the bag, you may want to abandon, then it's not pulling the plastic from your seat and you should try something else.