I haven't tried them but they apparently work for people. I used to have a dog that eventually wound up salivating and vomiting even going over to the car. I don't know about your dog but I think it can end up almost a conditioned response. If that is the case with your dog maybe slow and systematic desensiting (get in the car first without driving and reward, back up the driveway and return with reward, to the end of the street etc) might work if you are patient enough. If you reach your wits end you might just be better off travelling with him in a wire crate that can be hosed off, at least it will save your car interior. It can be a real pain, I know, so good luck with whatever solutions you try.