Does anyone know if those flipdown lcds made for cars will work for a diy projector?

In theory any LCD would work. You'd just need to be aware that the resolution is probably SD (640x480) and that you're probably limited in what you can do with it.

Power is probalby going to be a problem, as it's designed for a car.

However, an LCD is an LCD is an LCD. Aside from wiring issues, it's always worth a try.