I will be tackling this this weekend and decided checking this thread out would be helpful. I'm glad I did because I got some good ideas. I was just going to install a switch from a constant 12V supply and do things that way but, I do like the idea of the lights coming on with the dome and fading out. I want to be able to do both so I'm putting in a 12V relay. I'm wiring the positive coil to the grey wire so when that comes on it will energize the relay and turn on the 12V to the inverters by switching on the normallly off contact with 12V on it. The normally on contact will also have 12V on it but, I'll install a switch in between the 12V source and the normally on contact. That way, the lights will always have a 12V source supplying it. One that I control. And the other that the dome light controls. Best of both worlds.
I'll just install the relay under the dash somewhere. Probably around where I install the inverters. Now the question is... where do I install the switch. hmmmm????