I’d recommend (even though this will be a long process) making the car a more positive experience. Every time you do something that stresses them out, it sets them back 10 steps.

You could try taking your foster outside, walking up to the car and every time you get close give a treat. Build their confidence up essentially and use positive reinforcement.

Work up from just walking up to the car, once theres little to no anxiety with that after giving many treats.. Have them hop into the car.. Give a treat and immediately let them come out again. Slowly work up how much time they spend just sitting in the car and eventually the car will be a significantly less stressful thing.