Mineral spirits.

by Andrew , Bloomington, IN, Tuesday, November 10, 2015, 19:47 (3304 days ago) @ stonewalrus

Not lacquer thinner, but plain old mineral spirits are great for removing sap, road tar, and general gunk from vehicles. Try a small patch first to see if it is effective. Follow it up with another wash with hot soapy water.

Your 1993 truck should have a base coat/clear coat paint job, but the clear coat on top will typically be fairly weathered by now, especially on the horizontal surfaces, creating a surface that is very receptive to the tars and sugars of tree sap and will hold onto them very well.