had a lil convo with zach. He said even if you tap into cabin light power and use it and use two power input (cabin & switch) for invertors , they wont dim (like cabin) coz they will only fire when they have 12v given, below they wont turn on.
But yes they can be turned on from 2 sources and for that there is no need for diodes, just a relay would be enough...
mentioned some pins to be adjusted, check the links that i pm'd