Joe,
My hardtop has the same problem. The taillights, and turn signal functions work just fine on the right side. However the brake light function on the right side does not. I've replaced all three bulbs, but still no brake function on the right side only. Because the other lighting functions on that side and the left side work properly, that suggests that it is not a ground problem. One of my Buick friends thinks that the problem is in the wiring harness that is part of the tilt steering column. Says that it is not that uncommon. However, I can't track it down. My limited knowledge of electrical issues is my problem. I hope you can solve it and let me know what you did to fix it. Good luck!
John E.