The coolant manifold Tone referred to, is located under the intake manifold.
It houses the thermostat, & also feeds or returns (can't remember which way it flows) the heating system.
There is a metal pipe connected to it, that is sealed with an O ring, that degenerates with time.
It's also REAL easy to nick upon reassembly, & you won't know you've done it until EVERYTHING is all back together & you're trying to add coolant...