PPG DX330 - Wax and Grease Remover
It's actually used to wipe cars down before painting, but I've found it to work exceptionally well on stickers. It's approximately $30 gallon, but if you're going to be doing any painting of parts or cleaning of tar off of your car, it's well worth it. 👍
EDIT: It is essentially paint thinner, but not as harsh of a solution.