I used to put my beasties in a harness in the passenger seat (with the airbag turned off). The harness works great if the dog settles down (I have one from dog.com that I like because I could also use it as a walking harness when we stopped for potty and I also had one that is designed with human-style seat belt material but it's way to heavy for our little basenjis) .
But Gossy became so excited about going for rides that she turned into an uncontrollable berserk devil so I finally bought a crate for transport. She's still a tasmanian devil but at least confined. Additionally if there is an accident the aide responders can just keep her in the crate until doggie help comes.
My 2003 LRT (little red truck) had a switch to turn off the passenger airbag. But I just bought a new Chevy Colorado extended cab - plenty of room behind the seat for 2 crates but the passenger seat is weight sensitive. So the airbag deploys only if a certain weight threshold is achieved and its more than 20 pounds so even if my beastie does ride up there the airbag won't deploy in an accident.
BTW - Gossy has figured out how to manipulate the zipper on the crate door so she can escaped from the crate while in transport this weekend. I'll have to get a little lock for the zipper. Little devil :p