unfortunatly, lacquer thinner will soften the urethane to a point, and dull the finish. i could go on for hours about paint chemistry, but i won't.
shawn, there's an old trick used in the custom paint and lettering industry to remove old hand brush lettering. this only works for air dry paints and not chemicaly crosslinked finishes. i would assume this to be an airdry paint. either way, this will not harm the finish of your car. try a spray of oven cleaner. it will soften the air dry finish to the point where it will damp rag off. do NOT get this on any unpainted plastic part.
let me know if it works, or call me with any questions..........