Model A's and B's are not known for effective brake lights, and even optional dual rear lamps are sometimes insufficient!
This is the best 6 volt LED center brake light I have found. It is actually made for use on heavy duty truck trailers which often suffer from bad grounds and low voltage conditions. It is a completely sealed and regulated LED device that operates on as little as 3 volts, and at full luminescence from 6 volt to 16.5 volts.
It is Federal-Mogul Signal-Stat part number 62000. It is quite expensive at a list price of $81.28, but is very nice and very bright. There are many other LED brake lights around, but they are designed to operate at 12 volts, or at much reduced output at 6 volts. This one is specifically designed for full power at 6 volts.
It is an oval shape, 2-1/4 by 6-7/16 inches horizontally oriented. I made a bracket and mounted it in front of the spokes on my rear spare, so it is hidden from view and not evident except when I step on the brakes.
Please see this link about original right hand side rear 'tea cup' lamp brackets on Ford Garage.