KRC4 Compact IO wiring

I want to use the robot IO to control a solenoid valve, we're using a KRC4 Compact.

The default IO mapping for the EL1809 is already mapped and working (checked the lights when I fiddled with the outputs on the pad) but I'm unsure how to do the physical IO wiring.
My solenoid valve has two wires, positive to go to one of the ports on the EL1809, while I understand the negative would go to the EL1100?
But of the three negative ports on the EL1100, two connect to each other, and the last connects to X55 (which I vaguely recall got connected to itself (to close the circuit?) by the kuka guy when we were setting up the robot years ago.

In this case where should I be plugging the wires in to?
(sorry if I'm missing any information/context, very weak in terms of hardware )

tl;dr For my valve, where should I connect my negative wire?

edit: added photo

Is X12 the one with 30+ pin holes?
Someone must've mixed the labels around as I've been thinking that was X69 all this time..
(and was wondering why it didn't match up with the manuals...)

But yes, I'm connecting to EL1809 through the 30+ pin hole interface, not directly.
The common ground thing is the thing I was stumped on, but I've since figured out I can take it from X55 (right?) pins 6/8?

The WV is already configured.

got the ground from X55, IO is working
would have gotten it sooner but had the pins for X12 mirrored (was stupidly looking at the labelled numbers rather than where the wires were actually going)

Thanks for the info about X12 though