Death Valley (1982) on Netflix

Plot summary

Sally has separated from her husband Paul, a professor at Princeton, and she and Mike, a boyfriend from her high school days, have started seeing each other again. She brings her son Billy from New York City to her hometown of Phoenix to meet Mike, and the three set off together for California. Mike and Billy, a precocious boy, get on each other's nerves while Sally tries to draw them together. Out on the empty desert highway, a weathered gold Cadillac goes past. At an abandoned mining site, Billy comes across a recreational vehicle holding three murdered young people, and he leaves with a gold frog pendant he finds there.