Hi, I am from Australia and have a 2003 Holden Vectra C z22se 2.2l Hatch - leaking coolant from the engine (between the firewall and engine) - I have done some reading on various forums and on older models at least it would seem to be the heater tap valve - I have rung a (gulp) Holden dealer in Adelaide and they were unable to find a heater tap valve part reference for this engine/model.
My question is - does this model/engine have a heater tap valve, if so, is the likely cause of the leak in your measured opinions? I am losing approx 1.5l of fluid each time I run the car for more than 30mins (on a 30 degree + day).
The pic below is (I believe) the heater tap valve for older model Vectra's (pic from Ebay)