Oh, I finally finished Amber Chronicles and I think I know what idea Jim will use from it.
Merlin, in the Amber series, has the car that he like lots on Earth. Anyways, toward the end of the series, since he has such an attachment with it, etc, it becomes a stopping off point where he brings allies to(teleport to in shadow), fills them in, continues on, appears there with a new set of allies with some of the previous ones remaining, others gone, they converse, etc. This gets repeated a few times as Merlin goes about his mission. When he gets to a new situation, something like this occurs at some point. Anyways, another issue that is involved is pattern-ghost or logrus-ghost. These are either constructs created by pattern or logrus, something kinda like a shade. Anyways, sometimes in certain situations when Merlin gets sent on 'ghost quest' by the cosmic powers of order and chaos, he befriends some and they accompany him and become some of the allies he brings with him to the 'car.' These allies then can interchange with their real selves or not. For some of these ghosts or shades, Merlin has to feed them his blood because they can't operate for long.
With this idea, Harry's allies can get clued in once Harry gets to this position and this 'car' need not be 'static' to time/place. The 'car' is on a higher plane of existence so the clued allies can be dropped off somewhere on the timeline and perhaps even along the parallel worlds adjacent to Harry's own.
At that point, the "there you are" could be at times some 'secret' handshake out in the open.
This FULL 'CAR' ability would be like Harry having access to Uriel's office in Ghost Story. And a car that drops people off.
The small 'car' effect could work at Harry's gravesite, or the basement of BFS building maybe. or demonreach. Any site though would probably be a nevernever side of things.
