This was an odd rain leak, which turned out to be due to an unusually-thick bead of factory seam sealer below the heater nipple penetration (blue circle) that prevented the outside HVAC box gasket from making a seal. I chipped away the sealer, added TackyTape to the foam, straightened the inboard seal (with the orange pointer, which didn't seem to be leaking), and reinstalled the box.
While the box was out, I also pulled the exhaust manifold, had it planed, and reinstalled it.
IF THE IMAGE IS TOO SMALL, click it.