To find out where the audio channel cables connect to the audio player, you must have the data sheet or circuit diagram of the player.
Usually you can easily find it in google.
I found mine in Google and if you can't find it in google sometimes you can find the channel's label on the PCB itself, or you can use a multimeter and know the IC model you can find out the channel's pins.
each audio source usually has two channels left and right and as I mentioned that I will connect mine to the AUX source, I searched the datasheet for AUX LCH and RCH.
so now that I found the channels for the channels I will connect my Bluetooth adapter channels to the pins on the audio player card.
I soldered two wires to the IC pin groove.
BTW each channel from the Bluetooth adapter has two wires white and red, the white one is for signal while the red is for ground, so I connected both red wires to adapter ground and to the audio player's case which is ground. and connected the white cables each to the cable I soldered to the L and R channels.
Last but not least, I mounted the Bluetooth adapter to the audio player's cover with dual side glue, but first you need to make sure that there is enough space above the player otherwise it will mount somewhere else.
now the only cable left is the 1