I'm going to guess that something that you're unaware of happened to him either in the car or in one of the places you have taken him.
In any case, the only way I know of to rectify it is to make getting in the car a pleasant experience again, using food toys or something else he loves as a reward. You'll probably have to set aside special times to "train" him to get in the car again. I would expose him a little at a time to the car.
The first day, just take him on a leash close to the car. Reward him for not pulling on the leash. Stand by the car for a while, while rewarding him. (I would use chicken or I hear roast beef is popular) Have him sit or do a few obedience commands.
The next day, open the car door, but don't even act like you expect him to get in. Reward him. The idea is to get him closer and closer every day without scaring him or pushing his buttons.
The next day, You sit in the car, but leave him on his leash outside the car.
You get the idea. Just move him closer and closer while rewarding him for not pulling on the leash. If he pulls, you're working too fast.
LSTM (Laughing Silently to Myself)
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