What is the test for a brief investigatory stop of a vehicle by a police officer?

A brief investigatory stop of a vehicle is constitutionally permissible if the officer has founded suspicion, i.e., "specific articulable facts, together with rational inferences from those facts, that reasonably warrant suspicion that the (vehicle) contain(s) aliens who may be illegally in the country." United States v. Brignoni-Ponce, 422 U.S. 873, 884, 95 S.Ct. 2574, 2581, 45 L.Ed.2d 607 (1975). Some fact or facts must focus suspicion on the particular vehicle as being involved in criminal activity. See United States v. Carrizoza-Gaxiola, 523 F.2d 239 (9th Cir. 1975).