Mark,
You could try using alum foil on the two taillight housings you are trying to brighten up on each side while leaving the taillight only as is. Foil pressed into place without touching the actual socket effectively re-silvers the housing for better reflection, or increased reflection as you desire. You will have to take the lens off to do it, so it will be a bit of work, but the final effect may even out the glow of taillights. It will also increase brightness of brake lights which is always a good thing.