2007 Linclon Town Car wiring help needed

[alum]

Hi there, I'm new here and need some wiring help for my 2007 Lincoln Town Car Signature L with factory THX sound system with Navigation.
I want to replace my factory head unit with one with USB input but hopefully keep the factory amp and subwoofer amp.
I apologize for not knowing any of the terminology.

I believe the factory head unit is a Pioneer. It does not have rca outputs.
It has 5 pairs of 22 gauge wires (shielded) running to the amp in the trunk made up of FL+ FL-, FR+ FR-, LR+ LR-, RR+ RR- and SUB+ Sub-.

I have a dual head unit on hand that I will use to experiment with.
It has speaker wires out as well as Front and Rear rca's and a Sub rca.

I'm guessing the factory head unit is just a pre-amp so I can't run the speaker outputs directly from the new head unit to the amp.
Is there a way to convert the rca pre-outs on the new head unit to speaker lines that I can connect to the original wiring?

Another option I was thinking of was a Scosche OEA-4 Factory Amp Interface.
This looks like it will match the speaker line output to match the amp.
That doesn't help with the Sub pre-out though.

Any help would be appreciated, thanks!

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