The oem audio system uses balanced signals between the head unit and the amp.
"What is Balanced Audio?
Balanced audio is a method of minimizing unwanted noise from interference in audio cables. The idea is that any interference picked up in a balanced cable is eliminated at the point where the cable plugs into a sound mixer or other equipment.
Balanced audio works on the principle that two identical signals which are opposite polarities (often erroneously called "out of phase") will cancel each other out. The cables used in such systems are designed to carry two versions of the signal and manipulate the polarities of these signals to eliminate noise.
This will make more sense when we look at how balanced cables work, but first we need to take a step backwards and look at unbalanced audio cables."
The outputs from your new head unit are probably simple line level outputs. It won't work with the oem amp.