Depends on your setup. There's a few versions. If it's anything like mine, there's no replacement other than OE, as the head unit and the amp are like one unit split in to, with a data signal between the 2 boxes. I.e. audio adjustments happen in the amp, but controlled by a proprietary digital signal from the HU.
This is on a non-navi, 2004 MY version, EU-market HDJ100.