1. Buy a toy play with the dog so its associates the toy and you with joy and throw it in your car. Try it a few times and see what happens.
Also calming herbs or oils will help like > essential Rosemary and Bergamot (and its good for the fur).
I think also the frequencies remote to be used on the dogs dna.
2. The first thing that came in my mind was: you may help your dog with Bach Recue Remedy, and maybe some other Bach remedies that fits later on.... I myself have good experiences with bachremedies and my dog.
You may check the following links to get an idea:
3. Before learning to walk on leash or get in your car, he has to learn to trust you. He's been living on the street for who knows how long and probably fears people to some extent. You were lucky he let you get him at all. The remedy you seek is gentle patience, time and love.
For more details, please check the link: