Your message will hopefully carry much weight, thanks to your personal involvement with the animals who suffer as a result of the stupidity of people who transport dogs in the back of trucks.
I have confronted untold people who transport their dogs this way, and to a man, each of them insist that "Oh, he likes it there". To which I respond: "O.K., if it's so much fun, you sit in the back on a rainy or blistering hot day and I'll drive you around for hours". I simply receive a curse word and a dirty look while they drive away with the poor animal still suffering.
Sadly, the mentality of those who continue to carry their dogs this way is such that they will steadfastly refuse to see the harm, possibly until their dog needs veterinary care for its injuries.