What's going on in the car is a lot of unspoken interactions. Matt being cut off from his mother, Alan's feeling that he needs to take control of Matt's behavior but not having the authority to do so, Rachelle's debilitating grief and reliance on the grief counselor to get her through the funeral. And how Alan and Rachelle's new start is completely at odds with what is happening in the family at the moment. Every one is lost in their own little world, unable to connect, and rubbing up against the other worlds. Then there's the juxtaposition against this image of Denny in the darkness of his room - a cave (or womb) where he has retreated just weeks before his death (which is an echo of the image of Rachelle in the interior of the limo waiting to go to the cemetery).
I see a lot of this novel dealing with people trying to connect with others and being unable to do so - and especially this sense that everyone wants to keep their hands off Matt, that if they leave Matt alone he'll do just fine. Which is challenging, because the novel then turns on moments of missed connection and I have to find a way to make them dynamic, full of subtext, and NOT melodramatic.