It was good to have good information.
The captions are "LAPD patroling with a police car parked at the intersection of Broadway Boulevard and Olive Street in downtown Los Angeles at midnight" and "Midnight Broadway Boulevard, LAPD to guide bad boys".
The magazine containing this photo was released in Japan in 1978. And the photo was posted on the special page of the experience work actually done at LAPD by Mr. Haruo Mizuno who supervised the magazine. Unfortunately, there are only two pictures of Plymouth, but it seems to be a real thing, not a movie or TV drama. Perhaps the light bar was introduced on a trial basis even before the 1980s.