Have you tried just putting her in the car and sitting with her for a few minutes without going anywhere? Then take her out of it before she has had the chance to soil, give lots and lots of praise and then do it again a little later, repeating it two or three times a day until she gets the hang of it. Once she is used to just sitting in the car and not going anywhere, try starting the car up with her in it. It may take time and perserverance on your behalf but it will be worth it if it works for your girl.