Wrong, given consent to search, law enforcement can search anywhere in the vehicle unless specifically told/asked not too, and you can change your mind at anytime and ask them to stop searching, at which point they have to stop...
What you are refering to is a search based on probable cause, or search incident to arrest. In that case, you can only search places accessible from the passenger compartment, with the exception of locked compartments (ie locked glove box). To search anything else a warrant would have to be obtained.
There are a lot of other specifics/details/exceptions, but I won't get in to all that
