What is the test for a warrantless search of a camper shell?

Appellants contend that the warrantless search of the camper shell was not supported by any exception to the warrant requirement. Respondent, on the other hand, asserts two legal theories to support the search: (1) that the search was made pursuant to consent obtained during a valid investigatory detention; and (2) that the search was a valid, warrantless search of an automobile based on probable cause. (United States v. Ross (1982) 456 U.S. 798, 102 S.Ct. 2157, 72 L.Ed.2d 572.)