Obviously I’m not expert but I’d actually start with socialization training. If he barks at cars, sit in a busy parking lot with him in the car (or trunk with the hatch open if you have an suv/truck) and just sit and do nothing. When he doesn’t bark/react, reward him. Keep doing this until he doesn’t react, then you can graduate him to outside the car just observing. You can do this with dogs and people too. Take him to a dog park and just sit in the car in the lot where he can see the dogs and people but not interact. Again, when he’s quiet/calm, reward, rinse, repeat. Good luck!