Not sure this will be much help :S
But I know SD use different wire colours to BKP "I think" but I'm sure you have probably figured that out when you fitted them.
Any who I had that hum problem with a BC rich I was fixing, not exactly the same I know. But It turned out it was a ground wire wired wrong :)
Not sure what to suggest, but check the toggle switch is good and the volume. Something might have even blown out, defo where a multimeter would come in handy :)
Sorry I couldn't help more