Thanks for the replies.  Luckily the bulb inserts and light assemblies
are electrically interchangable, so when I swapped them from left to
right I saw that the problem moved with the light assembly.  I was
getting voltage when the brake was applied, too.  More careful
inspection revealed that the metal prong on the light assmbly was
recessed (looked like the surrounding plastic may have deformed due to
heating).  I was able to pry it up with a plier and get the bulb
insert to make contact.  Brake light!!!   My concern is whether this
will repeat or be a hazard because it may have shown signs of
overheating.  In your opinion, is this cause for concern?  Thanks so
much.