Its a long slow process, but desensetisation can work. First step, put dog in car, DO NOT GO ANYWHERE OR EVEN TURN ON ENGINE get dog out of car. praise and reward. Do this until dog gets into car happily and doesn't dribble. Then extend time sat in car with you there always extend time slowly don't push it till dog gets wound up and give loads of up beat but calm praise and reward. Once all okay , start engine, turn off praise dog. extend engine running time then when okay with engine running, drive car one foot, stop. praise dog, etcet very very gradually extending driving time. Your vet can help with mild sedatives but our dog who had firework phobia used to be knocked flat by these so I personally would not recommend them. I have found that bach flower rescue remedy helps generally for nervy situations. You will need to take it slowly and persevere. Your dog is lucky to have you an (ex) friend of mine used to throw the dog in the car onloads of old towels and said the poor thing would just have to get over it or suffer!!