Yes, I have had this and the vet said it was fear rather then anything else. The advice we got was to adjust him very gradually.
We started by letting him sit in the car by himself but not going anywhere, he would fall asleep and relax. We did this over a ten day period till he was comfortable about that.
Then I sat in the car with him daily for about a week or so , a couple of times a day,and ran the engine for a little while,but we didn't move, then we got out, he accepted that.
Then we drove to the end of the street and back, and very gradually we extended the trips.
We had a special toy that he took, but only for the car and he got a very special edible treat when he got out.
He quite quickly connected the things in his head and after about three months he wanted to go everywhere.
It took a while but it really did work.
XX Jeannine