With the case open, the drives removed, and looking at the rear connector to the rear left (display closest to you), You will see the two white headers for the wiring to the main connector.

In front of the left connector (nearest the case wall), there are three rows of resistors.

R73 (left channel ground link) is the second in from the left, on the row closest to the white connector. It is marked "000".

R74 (right channel ground link) is the last resistor of five on the right, next to a capacitor, on the third row from the white connector. It is also marked "000".

I have no way to photograph these, sorry. I would recommend you measure the resistance between the ground sheath on the RCAs and the case first, and if you dive in with the soldering iron, then BEWARE that you have identified the problem AND the correct components.