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No commodore factory head-unit has "sub" outputs, especially not RCA or low level outputs as far as i know.
You have two options if you want to add an amp and sub.
1) Buy an aftermarket head-unit.
2) Buy a line out converter (LOC) which will convert speaker wires into RCAs, just connect the LOC to the rear speaker wires and then connect the RCAs to your amplifier, when you fade to the rear it will fade to the rear speakers and subwoofer/s.