how do u take stickers off a plastic body??? there stuck on hard??

i bought a pre-stuck-on sticker subaru body, and felt like taking them off, i filled up the bathtub, and put......uh something in there, like soap or laundry detergent, and left it over night, i dunno about doing laundry detergent tho, im not quite sure if thats what i used, but wutever i did use, after the night, the stickers peeled right off.