Green’s Dictionary of Slang

(US black) a machine, either a car or a motorcycle.

1968		‘Sl. of Watts’ in Current Sl. III:2 41: Sheen, n. Car.
1969		B. Beckham My Main Mother 9: The bad sheen parked next to Uncle’s station wagon.
1971		H.E Roberts Third Ear n.p.: sheen n. car; automobile (probably derived from machine).
1980		in S. S. Terkel Amer. Dreams (1982) 164: My friends are always talkin’ about havin’ a nice sheen. That’s a nice car or van.