This is basically because Airplanes use a 28V system, so the adapter can get the 15V it needs to keep the Mac alive, but in a car it is 12V and they can't bump up the Voltage without reducing the Amperage to the point that you can't keep the system alive.
What you need is an Inverter like
http://www.4lots.com/index.asp?PageAction=VIEWPROD&ProdID=95 which is 75W and all you need is 45 for the MBA.
TEG