True, not real safe, but I have carried chemicals behind the seat, with no problems. And I have carried bags of fert with crab control in the cab from time to time. Mostly sealed plastic bags. Rain safe and no need to unload at night. Most of the time I carried liquid herbicides in the bed of the pickup hidden down behind the tank. Sometimes secured with a chain and padlock through the handle. Rain--no problem.
Also I found a low plastic basket container (two inches tall and 12 by 15 wide) that just barely fit under the truck seat. It acted like a convenient drawer under the seat with a zip-tie to make it easy to get a finger grip on it. It was the right size for packets of Roundup Quickpro, Talpirid, non-liquid bottles of Drive, fungicide, and Manage. I could get about $400 dollars worth of chemical in the "drawer". I preferred it to be inside protected from rain and theft.
However, I should point out that a professor(fruit studies) I used to work with loaned us his faded green 55 Chevy pick up one day long ago, at Michigan State University. We never asked him again. Seems years ago, someone had spilled a bottle of Malathion in the front seat. It smelled BaD! Windows open--in any weather!