Lacquer thinner. This is the best solvent you'll ever use. It will completely remove old hardened pine sap to fresh gooey sap. Easily. BUT, two words of caution. Lacquer thinner is powerful stuff. It will instantly remove latex paint and dissolve some materials. I have used it with success on fiberglass, clothing and dyed leather. But dab a bit in an inconspicuous area first to make sure it won't dissolve the awning fabric. Secondly, lacquer thinner and it's vapors are extremely flammable. It evaporates very fast and makes gasoline look like child's play. Don't smoke or use it anywhere near an open flame, spark or pilot light.
Good luck,
John