The backlighting on my '78 CB750K gauges isn't working. I replaced the bulbs as the easiest first fix. No go. Then, while riding at dusk, I see that the backlighting comes up and operates/blinks with my turn signals!

I'm quite sure that it's the gauge lights, and not just light from the signal indicators.
How did this happen? What did previous owner do to create this? How do I fix it?
I'm an electrical dummy, and lost with wiring diagrams?
Help?
