I had this problem with my Brittany when I first got her. She would drool excessively, and eventually she'd vomit, especially if the ride was very long, or on rough roads. What worked for me was just taking things really slow. I started out by just taking her out to sit in the the truck in the driveway (with the engine off) for about 20 minutes each day. After she got used to that, I'd take her out to sit in the truck with the engine running. Then, once she was fine with that, we'd start taking short trips around the block each day. While doing each of these steps, I would comfort and encourage her so that it became a positive experience for her, and after we got out of the truck I'd offer her a treat. Eventually she quit drooling and vomiting, and got to where she was eager to get into the truck and go for a ride.