It is very common for people to bring a snack with them into the car to eat as they drive. In some cases, one does not finish the snack before he arrives at his destination, and he therefore wishes to continue eating after he goes inside the home (or wherever his destination is). Halacha requires a person in this case to recite a new Beracha over the food before resuming eating inside. Leaving ones car and going into the home constitutes "Shinui Makom" a change of location which requires reciting a new Beracha. Since the individual had recited the original Beracha and began eating inside the car, he is considered as having established himself in that place, such that a new Beracha is required if he continues eating somewhere else. This applies even if the individual had in mind when he began eating that he would continue eating somewhere else.
Therefore, a person who had been eating as he drove should recite a Beracha Aharona before leaving the car (assuming he ate a quantity requiring a Beracha Aharona), and then recite a new Beracha inside, before he resumes eating.
Of course, this would also apply in the reverse case, of one who begins eating at home and then brings his food with him into the car. He should recite the Beracha Aharona before leaving the home, and then recite a new Beracha over the food when he gets into the car.
In fact, this Halacha would even apply to somebody who leaves his car for a few moments. If one had been eating in his car and he must then leave his car for whatever reason, even momentarily, he should recite a Beracha Aharona before leaving his car and then recite a new Beracha over the food when he returns to the car, before he resumes eating.
This is in contrast to a case of somebody who began eating while walking outside. (It is generally considered inappropriate to eat while walking outside, though this is not our topic here.) In such a case, the individual did not establish himself in any specific location, and therefore he would not recite a new Beracha if he goes inside or into a car and continues eating. "Shinui Makom" applies only if one began eating in a specific location, and then changes location. If one began eating while walking about outside, however, he has no established location and he therefore does not recite a new Beracha when he goes indoors.
Summary: If a person began eating in his car and wishes to continue eating at his destination, he must recite the Beracha Aharona before exiting his car, and then recite a new Beracha over the food before he resumes eating at the destination. This applies equally in the reverse case, of one who began eating at home or at work and wishes to continue eating in the car.
