Step 5: Observing
I thought that I should intercept either one of these sets of wires. The reason behind this was that the speakers were connected to only board #1 and board #1 was connected to board #3 and not to the cassette player board. So, for the cassette player to work the sound must travel from the cassette player board to board #3, then to board #1 and then to speakers of line out. Hence, I needed to target the two sets of wires connecting board #3 to the cassette player board.
But I did not know which set of wires carried the sound.
I looked closely and found that the place where the set with 3 wires was connected to the board, the boards were marked with plus and minus. There was no marking where the third wire connected the board and I assumed that it must be the earthing wire.
If you look at the photo you would see the reattached wires #5 underneath which I have placed a white paper (just to act as a background for the photo). You can also see the two sets of wires coming from underneath the board #3 and connecting with the cassette player board (on the upper right side)