They were big late 70s Dodges, maybe Coronets?
When I was finishing up engineering school over 30 years ago at the university of Illinois at Chicago which is just west of the loop, I used to park on the street across the Dan Ryan expressway from the campus, because I was cheap and wanted to save buying a parking pass for campus parking. By where I parked there was a place that provided cars for movies and shows being filmed in the area. I remember seeing blue and white Chicago police cars parked there that said Metro Police that I think were being used to film street scenes for Hill Street Blues.
Around the same time there was a lame show on TV called "lady blue" which was about some Chicago Policewoman, so those cars might have been for Lady Blue instead, or maybe both shows.
There was a forgotten B movie that came out around 30 years ago called "One More Saturday Night" with Franken and Davis as hippie band members in a teen coming of age type thing. They drove around in a beat up old flat fronted van like an Econoline with Smashmouth or something spray painted on the side. I'd seen that goofy looking van in that prop car parking lot so it was cool seeing what it was for when I saw the movie.
One episode of Lady Blue there was some creep with a new custom Ford van driving around molesting kids or selling them drugs or something evil like that. Lady Blue tracked the creep down being the hero cop she was, and after slapping cuffs on him, she pulled out her 44 magnum and shot the custom van, which, of course, as always happens on TV, it exploded in a huge fireball. In the flames you could clearly see, it wasn't the cool custom van the creep had been driving, it was the old van from One More Saturday night with a rectangular box welded on the front so it sort of resembled the new custom Ford van! Cheapskates!
Then there was the time in the late 70s when I was in high school and me and some friends went to Victory Auto Wreckers just south of O Hare airport, and saw stacks of wrecked Chicago police cars there. Seemed sort of strange to see all those cars junked, as they usually get repurposed to Taxis, until around a year later I went to the movies and saw this cool new hit movie called Blues Brothers, and then I realized where all those police cars came from.
Man I wish I'd had a camera with me then!
Sorry to go a little OT, but thinking of those Hill Street Blues police cars brought back some memories.