Difficult to tell from the illustrations but I'm wondering if the deck end of those 1/8" cables isn't TRRS rather than the more common TRS. That third cable appears to be TRRS and since the RCA ends are Input and Output unless both RCAs are using a common ground it would have to be a TRRS plug going into the deck. If that's the case it shouldn't be too difficult to fabricate a cable if the pin assignment is known. (That said I generally hate soldering 1/8" plugs due to the limited space and my poor soldering skillz.)