Well, found the issue but not the source/location of the problem... I pulled the lense off the passenger side cab light fixture and verified the bulb was good. Out of curiosity, I pulled the bulb to expose the conductors in the fixture and put a test light on the conductors. To my amazement, the first conductor I touched had power to it?! Sadly though, so did the other conductor... I'm no electrician but I think this means I've lost ground somewhere... So I pulled a lense off a fixture down on the belt line that was working to verify wire colors for power and ground, and then ran a jumper from battery ground to the supposed ground wire on the cab light fixture. Sure enough, all of the cab clearance lights popped on!!
So... I made a quasi permanent jumper and tied it to a ground location inside the coach behind the cabinets. Not sure where or how the real ground failed, but the lights are working again now.
