There is also a ground wire on your rear light pods which is separate from the light bar in the rear window. Make sure your grounds are good before you go further.
are you sure the bulb is good?? my '99 SL1 had the same problem, neither lower brake light would come on but the tail lights worked just fine. And it's the same bulb. I thought that since the tail light lit up, the bulb must be good...
Wrong.
When I actually took one bulb out, i could see that it had two filaments. One was broken, the other good. so i replaced it and the new bulb worked. Replaced the one on the other side as well and have no more problem.