To interface RCI-2950DX we’ll get mic and PTT signals from front connector (looks like GX16-6) and audio output from the speaker jack in the back (3.5mm TRS). This arrangement is similar to other cables for mobile radios (#1, #2) which can be used as a starting point for this build.
The cable pinout will look like this:
GX16-6 pin #1 → 3.5mm TRRS sleeve (GND)
GX16-6 pin #2 → 3.5mm TRRS ring 1 (RIG_AFIN)
GX16-6 pin #3 → 3.5mm TRRS ring 2 (PTT)
3.5mm TRS tip → 3.5mm TRRS tip (RIG_AFOUT)
3.5mm TRS sleeve → 3.5mm TRRS sleeve (GND)
It’s possible that this connection is also needed
GX16-6 pin #3 → GX16-6 pin #1