i had a Shep that wasn't trained. he was house broken and very friendly. he loved to ride in cars, anybody's car. if the door was open he was in. one day my neighbor came home with his new Caddie. there was some one in the front and 3 people in the back. my neighbor gets out. down the steps my dog goes and jumps in the front seat and continues to the back seat. now the people in the back are screaming and trying to get out of the car. he laid down on this woman who was really scared. i run after him and get him out of the car. the people now realize he's friendly. i'm apologizing but everybody is starting to laugh. the lady that was screaming tells me how she is afraid of dogs. she kneels down in front of him and let's him lick her face and she says "i like you, though". so it ended on a good note. when it's time to go for a ride now my boy does a loop around the car then sits in front of the back door. as far as training your dog not to jump in cars i'm not sure what to do??