I have some new information on two fronts:
After looking at the service manual for one of the Legacy/Outback OEM HUs (GX-201LE), I'm certain that pins 1 and 11 are the correct pins where the nav audio prompts are piped into the HU. The HU routes this to the front speakers when the nav unit needs to speak. As discussed above, the audio signal is low-level and must be amplified.
The 20-pin socket has several other pins used for the HU to communicate with the external devise(s): BUS-ON, BUS-OFF, BUS+, BUS-, MUTE, and RESET. If the nav unit uses any of these to decide whether the HU is present and ready (and thus, whether the nav unit should send the audio prompts), there's probably no easy solution.
However, I see that pin 10, labeled "B/U" (or "BU" on the HU's schematic) provides +12V from the HU to the external devices. The HU takes +12V constant/battery (from pin 6 on the 14-pin power/speaker harness), passes that through a 5 Amp fuse inside the HU, and sends that out the 20-pin socket.
So it might be as simple as sending +12V on that pin to the nav unit to enable the nav audio prompts. Certainly worth testing. The nav unit obtains its main power supply from the 10-pin socket on the nav unit.
What Hol described as a "standard female speaker socket" on the back of the nav unit is a black, 3.5mm jack. It is intended for Kenwood's KNA-SP200 active speaker. The jack has 3 pins, and supplies +5V to power the speaker. See pictures and discussion starting
here (on NASIOC), and pin-out
here (post #502 in that thread).
In order to use that jack with something other than the KNA-SP200, you must build a cable with a 3-pin 3.5mm plug. Do not connect anything to the tip. Connect wires to the ring and sleeve terminals. Those will have the audio that you need to send into an amplified speaker (or a HU's aux input).
But do try the pins on the 20-pin socket, too.
Hol- what model and year Forester do you have?