Look up the history of FM broadcast automation for more on how this worked - it's fascinating. Starting in the 60s, companies like Drake-Chenault started implementing these Rube Goldberg-esque analog systems that involved tapes with subaudible tones at the ends of songs that would trigger a relay system playing other tapes or cartridges with commercials on them. Stations would subscribe to a service that would send them new tapes with the latest Top 40, AOR, or MOR hits, and then they'd record carts for local ads, weather, and news.
See also: https://en.wikipedia.org/wiki/Broadcast_automation