I used Goo Gone. in a bottle like rainx.. given to me by a friend. A mechanic recognized it in the trunk one day and told his customer about it to remove tar.. he said it was the perfect solution. Since I didn't buy it I don't know where it's sold.

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Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)