Henry has a Sleepypod clickit-sport harness that he seems to really like. All of my dogs have used this brand and seemed very comfortable. It is the only one I found that has passed crash tests. In the MH he rides on the loveseat with the lap belt through the straps in the back of the harness. Shoulder belt provides the same in a car. I feel better knowing he is secured in event of an accident and it keeps him from coming to visit me while driving. Henry is a 65 lb golden retriever who would be right next to me in the cab if not secured.
http://sleepypod.com/clickit-sport