Consumers are piling into General Motors and Ford showrooms, and sales are booming. But the stocks of the two remaining U.S. car makers are in the breakdown lane.

Early Friday, GM was trading just above $30, versus the $33 it commanded when the company returned to the public market in November 2010, after emerging from bankruptcy protection. Ford Motor shares were at $13 Friday, close to where they sat three years ago.

The…