All depends on how you wired in the aftermarket head unit.
MOST of them have 2 volt preamp outputs, this WILL NOTdrive the OEM Infinity amplifier enough to get good sub woofer, or door speaker volume, you must use the head unit's speaker out wires to drive the amplifier.
If you got lucky and your new unit has 4 volt preamp outputs, then you should be able to use them.