Lots of short, repeated exposure to the car, with treats that follow. Maybe your parents could just sit in the car for five minutes and give him treats when he comes over to the car? Then, close the doors, and treat him, then short trips, treat him..all over time?

Jasper gets sick in the car. Ugh. So, we did lots of little trips- no treats though, as that only adds to his tummy troubles. He just isn't geared for long rides.