http://www-odi.nhtsa.dot.gov/home.cfm
For the most part a defects investigation will only be started if the problem is safety related...Like if your car died in the middle of an intersection or in the fast lane on the highway, something like that .
For a recall to get started you have to file a complaint, enough complaints and they start a defects investigation, from there it usually goes into a tsb or recall depending on the severity of the fix and the cooperation of the manufacturer.There are other phases of this process but this is the basics from what I remember.