I had actually thought about both as well. The heater core is a no go because one of the nipples is 3/4" while the other is 5/8" The issue is locating a brass or stainless steel branch tee for 5/8" that provides a straight through barb fittings in 5/8 and the branch be threaded. Remember we have a very small amount of space back there do adding a bunch of hoses and clamps is not something that I want to do. and also because of the heat the use of plastics is out of the question.
Now I also thought about the in and out lines from the turbo. These are 3/8" line with an m14 banjo ring that attaches to the turbo. So they are hard lines. To attach to these lines and make it "clean" I would have to use a 3/8" compression T and cut the lines. This can be done as well but my question is would it differ from connecting to the second set of in and out ports on the turbo?, This would be the cleanest way to go about connecting it.
I have to look at my original turbo again but I am pretty sure that the coolant lines were hooked up incorrectly from the factory. And as such I have them hooked up incorrectly to my new turbo.
According to BorgWarner who makes both turbos the cold coolant is supposed to enter the bottom of the center section and exit the top section. The lower radiator hose is where the hot coolant enters the radiator and the upper hose is the cool coolant. The nipple that feeds the turbo on the intake side of the engine is right next to the upper radiator hose connection. This would be the cool coolant. This line is attached to the top port on the turbo, which would be incorrect.
I can also validate the flow of coolant through the system because the heater core hoses are attached to the thermostat housing on the back of the block where the pipe from the water pump is attached This is where the out connection for the turbo is also attached. But the out is connected to the bottom port on the turbo I am pretty sure.
This is not an ideal setup for convection siphoning when the engine gets turned off. The coolant is not going to move and not help cool the turbo when the vehicle is turned off, This is going to cause the life of the turbo to be shortened. I am almost 100% sure they are connected to the wrong ports from the factory.
If someone knows for sure the direction the coolant flows through the system and would let me know this would help me out. If someone that owns a 2.0 can check their turbo and see where the lines are connected and let me know it would help me out as well. I will check in a few minutes to see what ports do not have plugs in them on my OE turbo.