This stuff works great for tar. Sprayit on, let it dwell a few minutes and then lightly wash and rinse off.
http://www.topoftheline.com/tarwaxremover.html
I have heard others using mineral oil, baby oil, WD-40 and kerosene. If the tar is on some of the lower panels then you may want to try one of these home remedies in an out of the way area and see how it works.
Anthony
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"The Art & Science Of Auto Detail"