The water pipe from under the dizzy goes to the passenger side heater matrix hose (Inlet)
The other heater matrix hose (outlet) goes onto the black big plastic pipe that runs from under the dizzy area around the back of the block and the 'T' pieced hose is 1 from the little pipe near the alternator area as the 16v (On a cav anyway) has 3 hoses going to the expansion bottle
the point of it is, gte16v bottles have 3 connections, a small pipe from the inlet manifold to the top of the header tank, a medium sized hose from the top of the header tank to the top of the radiator (passenger side of the car) and a second medium sized hose from the bottom of the header tank to the main plastic engine hose.
i'm gussing the T piece is to join the small hose from the inlet manifold to the medium sized hose from the radiator. for header tanks that only have 2 connections.
that small pipe needs to b just blanked off, if the engines out of a calibra/cav/late astra wiv abs then the water bottles have 2 return pipes. ive put cav engine in my early mk2 cabriolet and blanked that pipe off and runs fine. (the pipe is abt half inch wide) and wheres my unlock codes srstealth???