As your dog gets more and more comfortable, go for longer rides. Drive to places where the dog has fun. Take a short ride up the block to the dog's favorite walking area or to the beach, park or a friend's house. Let the dog associate car rides with fun, pleasant and rewarding experiences. Do not rush the dog into anything, or it may just slow down the process even further.
Some dogs associate the car with good times to the point of becoming overly excited.
In this case, begin taking the dog for short rides that do not end up at the park or beach, but back home again. Teach your dog some obedience commands like down-stay. Sit in the car with your dog and practice there. Start out on a quiet street. Obviously do this with the car parked.