Tube on the heater core feed pipe, which is that one circled. I think I'm supposed to route it like so (the blue line) -
From what I see it looks like it skips the all the hardpipe mess like that. I already also had the heater core bypassed because I was hoping that was what was leaking instead of these impossible to reach little tubes. Now that the plenum is off and the culprit has been identified, I wouldn't mind skipping as much of the rusty old metal piping as I can.