Wiring Harness looks wrong
 

Hi, I am going to put a new HU in my car, I used to have the sc 816 but it stopped working, I bought a new harness for the double din head unit I bought, but for some reason, there’s only one cable connecting to the smaller plug, there’s also seems to be another set of plugs connected to the harness,
https://cimg7.ibsrv.net/gimg/volvofo...2da5a70bc8.jpg
The singular cable on the left

https://cimg4.ibsrv.net/gimg/volvofo...9a60a918f7.jpg
The other 2 adapters which where covered in old foam

There are no speaker wires connected to your head unit because - You have an external amp. Signal exits the head unit through the DIN cable, and the speaker wires are plugged into the external amp.
Wiring diagram below - note the single black and yellow wire in connector B on the back of the head unit.



https://cimg0.ibsrv.net/gimg/volvofo...cf6713b675.jpg
https://cimg2.ibsrv.net/gimg/volvofo...596e8b2392.jpg