I've got a 7 lb chihuahua that I bring along in the passenger seat up front. They make a mini-leash that will attach to your seatbelt, then the other end to your dogs harness. I just put a small blanket down so he's not on the seat and it works great. Its alot easier then using the carrier I have, and he's happier too since he can see what's going on (though he usually just curls up and goes to sleep).
This isn't the exact one I have, but close enough:
http://www.champk-9.com/CCSS.htm