It'* behind the dash between the stereo and glove box. You may be able to access it from the passenger footwell if you remove the hushpanel. It'* a round plastic manifold with a bunch of vacuum tubes coming out of it. The ECC unit uses vacuum to actuate the valves and doors. You might have to follow all them around.
The advice to check the fittings on the vacuum manifold under the hood is good, too. It'* the little black box between your black maxifuse relay center cover and the windshield wiper motor. Black box that looks like the maxifuse center, or part of it. There are 3 or 4 vacuum fittings on it that are prone to breaking. You can see this manifold in my sig pic. Directly above the strut tower, and to the left of the WS wiper motor assembly. Little black plastic box.