DMP3A vs DH100ix, Dension car stereos from the 2000s

I have only three pins of the 8 pin loom connected here, permanent 12V, switched live, and a ground. When the DH100ix is powered but off the DMP3A will spin up the disk, and then park it. The the DH100ix is switched on with the power button (or when ignition is turned on when it's in a car) both units wake and the DMP3A starts playing, you're hearing audio from the headphones plugged in ILLEGALLY to the DMP3A, I've got the drive caddy half dismantled, when it's correctly together the power and headphone sockets on the DMP3A are occluded. I need to find the data transfer lines between the units next.