I used to do car detail. What we did, to remove dealer stickers on trade-ins, was to use the power sprayer. This works for removing all kinds of dealer stickers. Including really old ones.

Note: For power washers, please use one at a car wash - please don't use the power sprayer that you would use to clean your siding... I don't want to get blamed for any damages. A concentrated sprayer from a garden hose may help to loosen the sticker... Then you could follow up with 3M's adhesive remover - may need to alternate between the two, if the sticker's being fussy.