Okay, I finally got around to pulling out the heater box from under my
rear seat. I pulled the old (leaky) heater core out and noted that
there is a (previously unknown to me) switch to control the coolant
flow through the heat exchanger. I unbolted this switch in preparation
to put it on the new replacement heat exchanger and noted that the
o-ring (?) was pretty darn stiff and was no longer o-shaped in
profile...

Not a big surprise for a 13 year old vehicle. Since my new heat
exchanger didn't come with that o-ring, (and no mention in the bentley
of anything special here) I put it to the list if there is anything I
should be aware of towards getting a replacement one? Otherwise, I'll
just visit my local flaps and get one appropriately sized.