There are two concepts for using aluminum in the body structure of automobiles. First, known as the spaceframe concept, utilises aluminum in extruded and cast forms, welded together, to form a body structure. This structure provides the stiffness and the strength and is covered up using formed sheets to give the shape to the body (Audi A8 uses this concept). Extrusions are of 6xxx series aluminum heat treated to T6 temper to get full advantage of strength. Castings are of 3xxx series.
The other concept mimics the way steel car bodies are made. In this formed sheets (5xxx series aluminum)are spot welded together to form the monocoque body.
Apart from Aluminum Association website, a lot of information can be obtained from various announcements/advertisements of aluminum companies. Most of them (Alcoa, ALcan, Reynolds, Norsk Hydro ....) had aluminum car body program going on at one time or the other.