<VV> MT Backup Lights

[Byron Comp]

Aside from a shorted wire, which I can't find, or a bad switch, which I don't know how to test, what would cause the backup lights on my 4 speed '64 Monza to come on with the ignition switch and stay on until shutting down? They didn't do that until just a few days ago. I unplugged the wire from the wiring harness in the engine bay for now.
 
Thanks in advance for your suggestions.
 
BTW, Bob Helt's book is a real eye opener. Don't know how to do everything I read yet, but I'm learning.
 
Byron Comp
'64 Monza Vert
Williamsport, PA