RAIN-X! i know this for a fact. i had a sticker on my back window and i took it off cause it faded and what not. but when it would get wet it would have the outline. so fast forward a few months when rain season, or month, came around i put rain x on all my windows. shortly after the 1st rain there was sun for about a month and it was hot so i gave it a bath and no more outline. i know it wasnt the rain cause thats just water. but it was the rain x and some scrubbing. works for me on all my sticker out lines. o and also i removed 2 little stickers i had. used rain x and they were gone too. after i cleaned all the residue too. so yeah! try that out.