Cut a 5" length of Teflon twisted pair and strip back the ends.
Attach one wire of one end to the lowest terminal of the headphone jack on the side nearest the volume pot.
Use your DMM to find the other end of that wire.
Route the twisted pair under the shielded input wiring and back under terminals 40 and 35 to terminal 30.
Solder the wire to the upper hole of terminal 30.