How to dig out behind retaining wall?
Oooo!  That's a challenging problem.

An idea that springs to mind is try renting a pneumatic gopher: variously called a hole hog, underground pneumatic piercing tool, air hog, etc.

There is a (crummy) Wikipedia article about them.

Those bore in a straight line, so I expect starting it some distance back from the wall and point it downward at about 45&deg; to miss the bottom of the wall.  You'd then have to dig a hole to find the borer.



Here I show digging a big hole at the left for device access.  Bore a hole for the new connecting pipe (magenta).  Dig a hole at the lower right to intercept the bore and provide access to connect to the lower line.  (I should have erased your red line.)