The Clinton Administration's grand scheme to help Detroit's Big Three develop a cleaner and more fuel-efficient car is finding the road bumpier than expected. White House techies had hoped to build the car with almost no new federal money by using defense and space research in exotic technologies such as lightweight materials and fuel cells. But guess what? Commerce Dept. technology chief Mary L. Good has discovered the cheap plan won't work. The promising technology is far too expensive for the auto market and will need costly R&D to make it commercially viable. If the Administration wants to make the project work, it's going to have to scrounge around for big bucks.