Trust me. The line that goes to the pump is the return line. You can see in this pic the routing of the lines as they go into the tensioner assembly (it's near the center of the pic laying against the strut tower).

http://img.photobucket.com/albums/v3...p/DSC02746.jpg

You want the upper one as it connects to the engine. No matter how they're connected at the heater core, that is the one that feeds it with hot coolant.