I remember back when I had my repair shops in the 90's, Neilson installed a box in some TVs, rather than having people do the survey. Most times the install on them looked like real crap, I never really understood why the people with nice sets would let them install the things.
If I remember right, the units connected to the phone line and transmitted whatever information was gathered. The people that I saw with those boxes installed in/on their TVs always seemed to be above average as far as financially, which made me tend to believe their ratings weren't the best because they seemed to cater to only select, wealthy people.
My aunt had that. They installed a separate phone line, which she had to pay for after they took out their equipment.