We originally suspected a bad thermostat-housing gasket and replaced it (it’s actually just an O-ring), but that didn’t stop the leak. The logical remaining alternative was a faulty intake-manifold gasket. Out of curiosity, we stuffed shop towels along the front and rear edges of the manifold to see exactly where the leak was coming from. In the morning, the only wet towels were at the rear of the intake. Removing the intake manifold isn’t difficult, but it did require removing the hood. That way, we could remove the whole windshield wiper assembly, which allowed much easier access to all the intake manifold fasteners.