Have you tried truck wash? A mate of mine worked next door to a car trimmers and this is what they used. You have to be careful as it is caustic, so dilute it as instructed clean area then wipe over with some soapy water. I tried it some squareback seat that were brown they came up white.
Screwfix sell small bottles but i used something similar from HSS which is 5ltr,i still have nearly 5ltr so try to get a small bottle.
http://www.screwfix.com/p/zenith-traffic-film-remover-2ltr/42326" onclick="window.open(this.href);return false;
Vinegar might work, but it might stink.