Thanks very much for your replies.
t01blaze, what I was thinking about was trying to splice in a third functioning taillight/brakelight/turn signal socket, in between the 2 existing taillight/brakelight/turn signal sockets, then cutting an opening in the back of the tailight housing, in between the 2 existing openings, and plugging that third socket into the new opening, so that instead of having just 2 tailight/brakelight/signal lights in each taillight assembly, you'd have 3. Another idea I have, is to put a third light there, but instead of having it as a brake & signal light, it would be just a running light, with the small peanut (#194) light bulb in it, and just splice it into the existing side marker light wiring that's already there.
As I said, just trying to think of something that's a bit different, without the expense of going with LEDs.