I've checked some of the "teenage runaway" movies from that era, and it's not Born Innocent (1974)
, Diary of a Teenage Hitchhiker (1979)
, or Dawn: Portrait of a Teenage Runaway (1976)
. This one seems to be a play off of those, but one where the hitchhiker ends up being the manipulative perpetrator rather than the defenseless victim living in a harsh world. A nice going against the grain of established cliche that a lot of those "message" movies from that era were guilty of, which is part of the reason why I really want to see this one again.