Thanks for the input all. Unfortunately I didn't get to look at it this weekend so I'll have to wait till Friday to check it over. To answer a couple questions:
"Taking out" means it burned out
It's a 12v 1004 double contact bayonet base bulb
When it burns out, it's black which would be over voltage - but it's the only 12v that's not playing nice
Checked voltage . . . 12.7
The rig has been on our seasonal site since April and I'm putting in bulb #3 - no traveling
A loose connection is most likely. Hopefully it's close to the switch or the fixture.