We had trouble too
First I would just take Mikey out to stand by the cars (as he would drag us past them or freak out and shake) -and then his reinforcement for 'cooling off' was cookies and no car ride. We did that several times an evening-randomly.
Then we went and practiced-getting in and out of the car-for treats. Then short car rides.
All the while of course we needed to do some car rides to get to the dog park-none over ten minutes-and always to a fun destination.
What really helped as well was taking him for a car ride with a good doggie friend of his-the friend rode well in cars and loved them and Mikey saw that and relaxed.
Just lots of gradual exposure to the car-especially when your not going for car rides-and give plenty of praise and treats (for good behavior only of course-you don;t want to reinforce bad stuff) and you'll be literally 'on your way'

hehe