Can't do it. They have no right to conduct any type of search. I got stopped last week as well for the same thing. The moment the officer asked me for my driver’s license, I told him here you go officer, but I oppose any form of search. When he asked why? I told him how about the rule of law. He just stared sharply at me and handed me my license back.
These check points are getting out of control. They do almost nothing in arrests.
The checking was a visual check. I was driving so I thot it odd they look in backseat for my seatbelt. It was a shakedown, pure and simple for whatever they might turn up, the "law" for my security mind you, gave them the pretext to stop me. I suspicion that had I had something of interest in the backseat, they would have "asked" permission to search.