We tinted our back windows quite dark, with a special anti-UV coating, and it's great for our kiddies on the back seats. Plus the car is not too hot when parked in the sun when the sun comes through the tinted back windows.
I couldn't find regulations for the side & back windows, but there are specialists who install film in Switzerland, so it must be permitted. Maybe give one a call, like these guys?
According to the law (see link), it is not totally clear (something not unusual in CH).
Full tinting of the front windows is a no-go but a band across the top of the window is allowed. From what I see in terms of usage, the driver/passenger windows are off limits as well. That leaves tinting for the back passenger windows and the back window.
Your installer will be able to give you the full story.
I passed an unmarked police car the other day on the motorway, Mercedes estate, and it's rear passenger and rear windows were blacked out, as in impenetrable jet black.
The blackness of the windows actually gave it away.
In the end I called strassenverkehrsamt and they said you can do what you want outside of dirvers view (so basically in the back), they said not papers are needed (unlike Germany and Poland).