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,

The singular cable on the left


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.