I know the SV6 doesn't have the 6x9's above the back seat, but do they run the wires for them? If so i was thinking i could run my in line converter from these.
nope, if the car doesn't have the speaker installed then it doesn't have the wiring there.
The easiest method i could see would be to pull the trims off the back doors and run additional wires from them speakers into your boot to then hook up the line converter.
For what it's worth no commodore ever came with 6x9s installed. They run an amp which powers two 8" subs...