69 Valiant Radio Help
After reading your post again, this is how I understand that you've connected things:
RED (dash) - YELLOW (head unit)
ORANGE (dash) - Illumination wire (head unit)
I don't have the wire diagram for your car, but it's entirely possible the orange dash wire is actually equivalent to a newer car's yellow wire (constant). I'd be surprised if these older cars had a dedicated illumination wire for future accessory add-ons. I could be wrong. If it was me, I'd try this:
RED (dash) - RED (head unit)
ORANGE (dash) - YELLOW (head unit)
Again, you didn't mention anything about a red wire from the head-unit but I've never installed one that did not have the red power wire.