Back in HS, a friend used to wonder about the possibility of a microwave heat/steam engine combination.
Was his name Orville Redenbacher by any chance?...
It would almost certainly be more efficient just to use the electricity used to create the microwave energy to drive the wheels. Was he going to use batteries to power the microwave? If he was going to use the steam to power a generator and to drive the vehicle, he would have a problem patenting it, since U.S. law does not allow patent protection for perpetual motion machines.
Interesting concept, by the microwave magnetron has to be powered by an electrical source, which means a generator. I suppose that a fire could start the water boiling until it could turn a generator to power a the magnetron, but the laws of thermodynamics chimes in and kicks over that sand castle with resistive and thermal and mechanical losses.