I used the new head unit in conjunction with the factory amp
4 wires from the original head unit split the sound left and right rather than left right and front/back. That’s why you only have 4 wires from the OEM head unit and 8 pin location on the amp

8 pin locations on the amp
You are going to have to split front and rear
The door speakers are regular speakers but the rear are subs. For me, I just wanted to use the factory amp with the double din head unit. With the roof down at highway speed it’s still fine. In a Coupe it would be pretty great. I’m 53 my definition of great sound and loud music are different from my son who is 21

4 wires you can connect these from the head unit through the Scosche/metro aftermarket adapter
Amp pin 6-Right door Speaker input (-)
Amp pin 7-Right door Speaker input (+)

Amp pin 1-Left door Speaker input (+)
Amp pin 2-Left door Speaker input (-)

My Kenwood unit has sub woofer out left and right. Yours should too.
The factory wires do not account for this.

I purchased RCA (phonographic plug) plug/Cable, cut it half and peeled back the wire. You could also purchase just the jacks and solder wires into the jacks. Purchasing ready made, we’ll, made more sense to me.

The wire to the center of the RCA Plug is positive and the shield (Outer connection) is always negative or ground. (Lots of illustrations of this on-line)

NEXT 4 WIRES are for the rear subs. Bypass the aftermarket scosche connector and run the RCA from the sub outs on the back of the head unit directly to the amp
Amp pin 9-Right sub Speaker input (-)
Amp pin 10-Right subSpeaker input (+)

Amp pin 8-Left sub Speaker input (+)
Amp pin 3-Left sub Speaker input (-)

You will have to cut and separate these wires from the amp. Leave plenty of wire to make the connection from the sub woofer jacks from the back of the head unit directly to the amp.

If you run into any obstacles, PM me and I’ll send you my phone number so that we can review over the phone.