The attached circuit (found using Google) is the sort of thing I remember from that era.
It connects to the speakers using a step-up transformer that also provides isolation (a small mains transformer, maybe 24v, backwards would do the job and have appropriate isolation). The filtering is basic but adequate to give some difference between the channels. It doesn't have any suppression and would probably need some adding if interference got back into the audio circuits.
The units sold for home use normally had a built in microphone so no direct connection to the audio equipment was needed. This is a typical example. Others would have had automatic level control rather than the pots.
https://www.electroschematics.com/so...rolled-lights/
In both cases the pots must have plastic shafts for safety.