I've got a similar, yet slightly more confusing problem on mine too!
It was laid up for 6 weeks, and I've just started using it again, and it's only got 1 brake light.....
The high level and drivers side brake lights work perfectly, yet the passengers side bulb WILL NOT light, I've tried 2 new bulbs in it, checked the white plastic bulb holder and all the metal strips supplying power look fine, the plug looks fine too, with no loose wires!
Any ideas?