Drive-In Massacre (1976) on Netflix

Plot summary

A couple park at a rural California drive-in for the night's movie, and a sword-wielding killer strikes them in their car. The next day, detectives Mike Leary and John Koch question Austin Johnson, the manager, and Germy, the custodian, who says a peeping tom cruises the drive-in at night to watch couples. Germy agrees to write down the man's license plate. That night a second pair of lovers is stabbed in their car, and a sword is left behind. Germy identifies it as not belonging to the owner's private collection and hands over the plate number, which leads the detectives to Orville Ingleson, who admits watching couples but denies the killings.