3 Answers

Get in the car without shutting the door and without the engine running.  Do this until the dog is used to the surrounding.  A little bit of reward could not hurt.

hold him, pet him reassure him. once he learns its not so scarey, hell want to ride wherever you go.

The other answers are on the money but to elaborate: Get in the car with the dog and offer small treats  and make it a fun places to be.

Do this for 10 minutes for several consecutive days then do the same with the car engine running, don`t do this in a closed area due to danger of carbon monoxide fumes.

Repeat but drive car a few hundred yards and stop, plenty of praise and before long you will have your friend blowing the car horn to go for a ride.

Whatever you do don`t force the dog to do anything, you will get nowhere.

Softly softly catchee monkey.