3rd Brake light...??

[1953M100]

Just purchased my 1st model A on Tuesday, a '31 Coupe.
My wife is a slight bit fearful and "sketchy" about safety of driving a slower, older vehicle. It has the aftermarket turn signal kit already installed, but one additional safety modification I would like to make is a 3rd brake light. Tried searching the threads here for "3rd" and "third", also been searching the interwebs, but would like some insight on making a good choice.

Questions :
- Several kits are "universal" mount, not finding any that are Model A specific. Where are y'all mounting these (back window, spare tire mount, etc)? What kind of mount did you fabricate?
- Most I'm seeing are 12 volt, I am assuming negative ground. Will these work by reversing polarity and running on 6V?
- Any specific kit or part # you would recommend...?

Thx!!