Once again...look at some of the older case Law....there are lots and lots of stipulations in regards to "Random" searches(ie...what does Random mean, every fifth Vehicle, every fourth Veh from New York, etc etc). I don't want to quote something in Legal jargon I don't quite understand but there is evidence that AG inspections DO, in Fact, help in keeping Calif(and other states) protected from AG born species foreign to ones AG industry. There is also case law regarding the manning of AG inspection stations and the authority for impeding traffic in general. Finally, there is standing for singling out those with "plates" from locations East of Kansas(I think that's what I read) for further investigation based on the "pest" that may enter from those locations. (Before everyone gets there panties tangled, Further Investigation would include asking one about Fruits or Vegetables and asking if a permissive search of a Vehicle or Trunk is possible.)
Just for the record...in Calif as well as every state in the US, there are provisions of the Vehicle Code, Business and Professions Code, Health and Safety Codes on and on, that can and will give a Law Enforcement Officer Probable Cause to continue an investigation....that's what those ticky tac provisions are for! In other words, play the attitude Game with someone who wants to make a Federal case about your refusal but be **** sure your NOT in violation of the thousands of other Vehicle Codes or any violation of Code that may be in Plain Sight because a CHP officer is only a phone call away. Yep, they may not gain access to your Vehicle/Trunk, but they can make life more difficult. My best advice and what I do is not at issue, Do What's Right for yourself but consider the many scenario's that could develop.