By replacing the existing single filament brake light and tail light each with dual filament sockets and bulbs, and installing two more dual-filament sockets and bulbs into the empty sides of the plastic reflector, it is possible to increase both the brake and taillight light output by a factor of four.  This modification also increases the lit area of the tail light considerably.  All sockets are wired in parallel: the brake light wire is connected to the same connection on all sockets, as is the running light wire, as follows:

Photos of the modifications follow:

side view          rear view (internal)            rear view - lit

The sockets can be bought at any auto parts store. You should get the shortest ones possible, to ensure that there is sufficient room behind them to reinstall the lamp housing in the tail of the bike. I had to cut a small hole in the inside wall of the tailpiece to fit the upper socket in place. I lined the inner surfaces of each lamp section with adhesive-backed reflective tape to give even more brightness. The final result provides a tail light that is hard to miss.