Same problem and a fix. (I have the convertible.)
I had a similar problem. When the headlights were off, both brake lights worked. I would occasionally see the "bulb out" indicator on the dash flicker or come on while I was depressing the brake pedal. I checked the brake lights and they seemed to work, although the driver side brake light was a bit dimmer than the passenger side. If my headlights were on, the driver side brake light would not work at all. The "tail light" part of the bulb would light up, but when I put on the brakes, the bulb would go dark. After a couple weeks of this problem, the driver side brake light quit working completely. I assumed it was the bulb. I replaced it but the symptom remained. I assumed it was the brake light relay. I replaced it but no fix. I started to worry that it was the ICM. Before I went down that road, I removed the driver side tail light assembly and closely examined all the wiring connections. It looked fine. Then, since I had it out of the car anyway, I decided to clean the bulb sockets. I cleaned the silver metal contacts inside the bulb sockets with a pencil eraser. I also used a small screw driver and needle nose pliers to bend the contacts in such a way that it would increase the pressure between the contact and the bulb. I felt like it was just futile work and that I was going to have to replace some expensive electrical component, but it actually worked. I reinstalled the tail light assembly and the brake light worked just like it was supposed to. The bulb sockets and contacts did not look dirty before I cleaned them. The contacts may have gotten a little bit shinier....gone from a brushed silver look to a shiny silver look, but basically it looked like I hadn't done anything.


I now plan on removing both tail light assemblies and giving them a thorough cleaning. Who knows, maybe it will get my 3rd brake light burning on full power?! (Right now it lights up but very dimly.)