Hmmm, the pictures reveal some differences to my original thought.
There is a voltage regulator mounted off-board (1.jpg, top image, right hand side - probably feeding 5V to the digital board beside it.
There are two separate output channels - red/yellow and blue/green (3.jpg, bottom image, two pairs of wires on the right hand side). I can't see where these wires go.... rear panel somewhere?
The 'input(s)' can be taken to/from the volume control. It's a twin-gang potentiometer and the 'outside' connections of each of the three terminals will be where you want to input a signal. One side will be ground, the opposite side will be the 'signal input'.
The red/black wires going to the back panel in your first post look to be a tap-off DC source for external use???
The yellow/green wires from the phone sockets...??? No idea where they are going from the pictures shown. Possibly not used?
It's all still a bit cluttered to get a proper idea of where wires go. The above is based on what I can glean from the pics as posted.
