solution found for annoyed wagon owners
 

(sigh) AGAIN, for anyone who needs to know, I found the REAL problem with cracking noises coming from the C-pillar area of the wagon.

It was the roof rails. I managed to move the headliner aside just enough to access the two 10mm nuts (perside) that hold the rails to the roof seam at the C-pillar, and loosened them to slightly less than hand tight.

First I tightened them quite a bit, but the noise became sharper and louder going over bumps.

The problem is definitely temperature related, in combination with an aging, fatigued, body structure, flexing front to back and diagonally a lot more now than when new.

Giving the rails just that much more slack to flex with the body rather than fighting it solved the problem. What a pain.