But it only happens with frequencies where one wave (a) fits in the car and (b) is quite large compared to the size of the human ear, so say 150-500Hz.

I seem to recall seeing the nodes around those ranges, yes.

All you can do about it is mount the SPL meter approximately where your head would be.

But my head moves. And I saw the nodes on the spectrum graph moving pretty significantly when I was moving the microphone short distances (just a few inches).

There might also be merit in averaging four (or two) runs, one for each occupant's head position.

Or perhaps just averaging four runs, one each with the balance and fader cranked in each direction.

The ultimate, of course, would be to get a system where it took advantage of the four-way mode of the equalizer and auto-EQ'd the rears differently from the fronts.