ok first off its not illegal, i check with my bro-in-law first before doing anything like that (yeah he is a cop) and as long as your brake lights work no other lights are required, what about cars without 3rd brake lights, do they have to go buy one and wire it up,,,,,NO they dont.
AFAIK, any car after 1984 *has* to have a 3rd brake light. I know in NJ the car will automatically fail inspection for a burnt out or missing third brake lamp. So, as far as I am concerned, it is law, for post-84 cars.