Keeping it real simple, below is the Joying harness, with the Amp-Cont cable out to the side. I actually extended this cable a little to give me more slack as the wiring is tight.
In a perfect world (i.e. if the Joying harness was a perfect match for the OEM 4C harness), the Amp-Cont wire would be wired into this harness second row from bottom, in the right-most slot. See pic for orientation:
Unfortunately the wire is open and does not have the right spade-type connector to neatly slot into the harness here. I tried googling around to see if I could buy a connector pin type thing but had no joy (I was trying to explain this in a previous post).
On the 4C factory loom (which the joying harness plugs into), the light blue/grey cable is the Amp cable which needs an ACC 12V signal from the Joying, to enable the amp to be switched on (so you get .. sound).
This wire is where the Joying Amp-Cont would connect to if it was installed in the Joying harness. As I couldn't get that to work, I used a tap
It's a tight fit, I would recommend using a small a tap as possible. You can see I've essentially bypassed the harness and tapped directly into the Amp-cont wire. It does mean there are extra steps to removing the Joying, but it keeps the car factory and OEM. Re-installing the Alpine would be a 20min job.
I then went around the tap and Amp-Cont cable with some OEM style cloth cable wrap to keep everything tidy and secure. Slotted everything neatly back into place, and went for a drive to confirm nothing was shaking loose.
Make sure to select the Amp-Cont option in the Joying settings. I'll spend an evening this week configuring the joying launcher to clean everything up so will try and take a photo of this too.
Hope this helps someone as I haven't seen this covered before in detail.