Robert;

I'm no expert on B16s, but pipe running from Wapu to back of B18 engine is the RETURN from Heater Core, by way of HCV...

Hot supply is from stubby pipe at back of Cyl Head, so if you're getting no flow at Heater Core, it's likely the Heater Control Valve is not allowing flow....but first, check Coolant level and assure it is adequate. Next, open" HCV and feel hose temp on either side to confirm flow. HCV and HC are plumbed in series, so any blockage in that path will prevent flow. Check and verify movement action of Temp control cable is really getting to HCV (sheath is not slipping, allowing lost motion).

See also: http://www.sw-em.com/Coolant_Distribution_Pipe_Notes.htm
Sw-Em tech article on Cooling Sys is not yet on-line, but it's coming...(so's Christmas!)

Good Hunting!