Here is another one to try. When i sold my small yacht it had white nylon sails with rust spots in them from being wrapped up with wet steel fittings. The guy at the slip yard sold me a small jar of "OXALIC ACID". The rust spots came out to my amazement after leaving this stuff for about 5 minutes on the sail and then scrubbing with a hard brush.
Maybe be totally different on a drive but you never know.