I think the heyday for stealing them was when cars first started having cassette players as standard option, and it became a big deal to have one.
Yes, I know that there was such a thing as 8-track players for cars, but they were never very popular, probably for 2 reasons: 8 tracks are bulky, and cassettes were so popular because you could make your own copies of albums, or you could make mix tapes for playing straight through in the car. People didn’t like having to buy an album for the house and an 8-track just for the car, but having as 8-track alone didn’t give them the quality sound of a vinyl album.
When CD players came out, very soon, the technology for the diskman and the car player came around about the same time, and so did the CD-to-cassette adapter, and so people used diskman players in their cars, and were happy with those until writable drives came out, then the demand for car CD players shot up, and those became the goals of thieves.
Now MP3 readers are the go-to technology. I have never personally installed one, but car manufacturers put the receptacles all over the place-- anywhere but the face of the players (which is where it is on after-market players). So if you do steal an MP3 player from a car, unless it’s an after-market player, it won’t do much good, because if you try to put it into a car not built for MP3, there’s no way to hook it up. The only customers for radio/MP3 players stolen as original equipment are people whose original equipment malfunctioned-- and that rarely happens. Without the moving parts of cassette players of CD players, radio/MP3 players last forever.
And what everyone wants is MP3 players. There’s little market for stolen CD players and none for cassette players, hence, no more stealing.
What was so bad about the stealing, and the reason radios have flashing lights, or removable face plates, or other ways to advertise their steal-lessness is that people sometimes broke windows to get them. Replacing the radio wasn’t such a big deal, but replacing the window was.
So that heyday was about 1980-2005 (maybe a little longer-- I’m going strictly on memory). You are right that it is over.