With the way the intercom is setup in our cars, there is no practical way to tie into the amp itself and or tie into the other mic and speaker. I am stuck with just the 3 wires going to the amp from my side.
Now is there a way to get the side tone without taping into the speaker B line? Somehow use my mic output (either before or after my pre-amp) through what ever black magic is needed and into my own speaker?
But if there's no way to access the "B" speaker(+) terminal, your a.m. black magic would be an external amplifier (plus its power supply: small battery) between your yellow mic(+) and green speaker(+), best with kak's 2-resistor + 1 poti solution to allow for volume adjustment.
For (a standard stereo plug), double plug adapters
are available, which directly connect all 3 electrical connections of the plug in parallel to the 2 female jacks. So you could insert your ZeroDegreeC part into one jack, and a custom-made amplifier (+ battery and volume control) into the other one.