I've had the switch go out on my D1 a couple of times. Since I have a 5-speed, the only sympthom is not brake lights.
If you disassemble the switch, you are likely to see two problems:
-- there are two sets of contacts in the switch. The one controlling the lights themselves is likely to have contacts that are pitted. Clean these up with some emery board or similar.
-- the little spring on this side will get fatigued. When this happens, the switch won't turn on and off properly. You can fix this temporarily by taking the spring out, squeezing it back together, and re-assembling.
-- Order a new switch, since the "fixed" switch will probably fail again in a few months (mine just did).