Not saying anything different than already been said above but here's a diagram which might get you closer if you're the visual type. If you (or someone else) have a diagram/photo of the internal wiring of the control box, the specific connections can be identified. I think you can even hook it up in "parallel" so that you can simultaneously control the accessory from the existing control box or the AIU...if that's of interest.
So you'd set up the accessory using 3 AIU Accessory Ports, in example above using ACC Ports 7,8,9. Label them something descriptive. Then when activating, use the ACT (rightmost) DCS remote button.