Green’s Dictionary of Slang

(US) one (of a couple) who uses a parked car as a venue for sex, intercourse or otherwise.

1929		M. Levin Reporter 388: In an hour the place’ll be full of petting parkers again.
1949		A.B. Hollingshead Elmtown’s Youth (1975) 313: As he [i.e. a policeman] opened the door he saw a couple in a sex embrace [...] He turned off the spotlight and left, justifying his action by the remark, ‘There is no use to run parkers out of town’.