<SNIP>

  ok, on the tail light fixture, it looks like this (driver's side for
example):

(top)
......................................
: yellow : clear : red1 : (license plate)
:.....................................:
: red2 : red3 : red4 :
:..............:..........:............:

yellow is the turn signal, always out the outside top of the fixture.
clear is the backup light. always on the center top of the fixture.
red1 is the brake light. always on the inside (next to license plate)
of the fixture.
now comes the fun part ... ;)
red2 is the driving/running/parking light. always on the outside
bottom of the fixture.
red3 is a sort of reflector built into the plastic.
and
red4 is just the bottom half of red1 ... both are open/connected
inside.

more even funner part ...
there IS a chrome-plastic reflector built into red1/red4 (brake light)
and yellow (turn signal)
to enhance their brightness.
there is NO reflector in the clear (backup) which is why backup light
at night is so crappy. :(
there is NO reflector in the red2 (driving/running/parking) which is
why it's so dim at night.
and there is NO reflector in the red3 IN THE U.S.A. (and no bulb in
the holder, not even
a bottom-contact tab for the bulb).
overseas, at least in the driver's side, red3 is a fog light and has
a chromed reflector in it.
but not over here. :(

hope it helps. good luck!
unca joel
<SNIP>A simple and cheap way to get better light out of a poor or corroded taillight reflector is to paint the area behind the bulb gloss white. On older, non-polished and chrome-type plated surfaces, that is the best work-around, as found by experimentation.