I don't think even I would find this one too difficult.
You could have the aircraft 12v supply to a USB converter, such as the Charge2/4, and then plug the Rosetta power lead into one of the USB slots. You could have a 12v DC to 5v DC USB converter (look on Amazon and eBay) and do it that way with the same Rosetta power lead (but might be noisy, which the Charge 2/4 specifically avoids) or you could have 12v DC to 5v DC step down transformer and wire up the Rosetta power supply to the 5v side. Nothing complicated at all I think.....and I can just about wire a plug.