Hi, had this with my cocker pup for weeks, its was terrified of goin anywere nere the car and once in would start frothing and stuff coming out of both ends within a minute of driving,
I did as already suggested with feeding in the car-- let them see their food in the bowl then they will probably follow you and let them see you put it in the car and then put them to eat it, I did this for a few days and also just sat the pup in the car standing near it building up to starting engine up etc,
It is something that they do grow out of especially once they realise that going in there mean