Turns out the Concours didn’t need a new bulb, it just needed some more electrical connection cleaning. After replacing the bulb that wasn’t blown I finally took off the fairing only to discover that, like all the other electrical gremlins, it was a matter of dirty connectors.
After cleaning up the wiring harness, suddenly all the lights work again. I posted what happened on the COG discussions and got this pearl:
As WillyP states above, bikes aren’t built to keep out the elements, even the most covered bike is virtually naked compared to a car. Even in the case of a well cared for, covered sports bike, cleaning the electrical contacts is a worthwhile off-season ritual. In the case of a field-found Concours, it’s where I should have started in the first place. A breakdown and electrical cleaning is my go-to next time around.
As a project bike the Concours continues to teach lessons even as it becomes more and more roadworthy.