Hello Pete,
Welcome to the forum.
Ken is correct the best way to go would be the auxillary board.
I can see that the top connection plug on the aux input/output connection is an output (more than likely to the sound to light unit) this uses pins 2 & 4 plus the ground at number 3.
I also that there is some other screened cables going below to maybe another plug underneath more than likely an input for CD with connection to pins 1 7 5 and again the common on pin 3.
Because of the length of the connection pins for the input, aux input/output and volume it could allow for more than one connector fitted, On the input & volume it was common practice to place a blank plug lower down on the connection pins to avoid shorts from screens on the upper connection plug if pushed to far down the pins.
Regards
Alan
Alan Hood
ami-man
UK