Mr Gerry, I think you are confusing the 'base' model coupe or roadster with the Limited or SRT6 models.
The base model has a different head unit ( easily identified by the black control ***** ) and no external amplifier or subs behind the seats.
As far as coupe or roadster, in either case the head units are the same, and they wire to the amplifier or speakers ( in the base's case ) the same.

If a replacement aftermarket head unit has 4 volt pre-amp outs ( the RCA plugs ), then they can be used to drive the Infinity amplifier, but if the pre-amp output is 2-3 v ( which seems common these days ), then you have to drive the amplifier with the speaker wires.