I would say that in nearly all cases the use of a passenger car would make it illegal to have ANY kind of pesticide inside the car, other than in the trunk. The wording of the law is that all pesticides must be transported in a compartment "separate from" the driver's compartment. Many companies use vans, and if they have built in partitions of some kind that clearly separate the driver from the storage areas then this may be okay with regulatory inspectors. However, carrying rodent baits in a cardboard box or a hand sprayer with chemical mixed up in it would not be considered compartments "separate from" the driver. Even a plastic rodent bait disc is a pesticide, and it must be stored for transport in some separate compartment. 

So, it is one of those somewhat vague regulations that is open to interpretation, unfortunately, but any county's inspectors may approve or disapprove of it. Obviously a consistent interpretation would be easier on us, but sometimes allowing things like this can be helpful. As a supplier of pesticides I believe that Univar has taken the stance that even the trunk of a passenger vehicle is NOT separate from the driver's compartment, and we will not release pesticides to people who come to our will call counters in their cars. Clearly air movement easily flows from the trunk into the seating area of that car, and thus any spill of chemical in the trunk could jeopardize the health of the driver or passengers. Just putting a locked box inside the car and claiming that this constitutes a separate compartment is a bit of a stretch, but do check with your own local inspectors and get their opinion on it.