The infinity amp expects at least 4 v PTP input. If your head unit is one of those 2.5 volt PTP units, then you have to use the speaker ( amplified ) outputs from the head unit.
You will get some distortion, unfortunately.
The preamp output is just what it means - the signal BEFORE the internal radio amplifier.
( PTP means peak to peak BTW )

Bottom line, do your research and ensure the unit you are installing has 4v preamp outputs