You do not need a warrant I don't care if you are US Customs or Mayberry's chief of police. This falls under the vehicle exception to the search warrant requirement as described in Carroll and hundreds of subsequent cases. Years and years ago.
That used to be true, but not anymore.
In
Arizona v. Gant (April this year),the Supreme Court severely restricted the warrantless search of vehicles, and basically undoing the Carroll Doctrine.
We had to do a whole bunch of search and seizure training in a big hurry after that case dropped.
I think this was a jacked up stop, and as a career LEO, I will not consent to search ever, anywhere, for any reason. I am sickened by the erosion in the civil liberties of citizens over the last 60 years, and will do whatever I can to beat back the tide.
I'm not beating on the cops, and no, I don't buy the 'dropped baggie', the 'dog trained to bark' theory, and all the other junk people have thrown around.
It's lots of other stuff, like TFRs, SFRAs, the TSA and their intrusive but useless security theater, the "virtual strip search' backscatter imagers, the constant fight to protect the 2nd Amendment, the nationalization of industry, and the death of the 9th and 10th Amendments.
It's not a cop on a stop, it's the daily intrusion of the government farther and farther into the private sphere.
If this story of a citizen stopped is the biggest government issue you see, then you're not paying attention to the right things.