....any ideas as to how to locate the underground junction?

Is the conduit metallic or some plastic? I ask only
because if it's a metal you can connect an oscillator of
some sort (GDO, Antenna Analyzer, etc) you should
be able to trace the route of the conduit using a pocket
short wave receiver, and if you are really lucky,
you might find a bend or some anomaly
that would point out the junction. I'm just taking a shot
in the dark, but it might be a quick and simple solution.

Irv VE6BP