That's the wrong side of the board to be useful, there aren't many ic part numbers visible. I'd start at IC501 on the left. Get the part number and see if you can find a data sheet on the web. It looks like an audio amp ic from the pin layout (although it might be a switching regulator ic). If an audio amp, I would trace the inputs back, and go from there. That's what I did with mine. Pictures and schematics of my HU mod on my site.
Also, there are aux-l and aux-r marked on the board, that would be a bit of a clue too.
Hang on a minute, just looked at some specs and this unit has a front panel aux in already. This is an absolute doddle to mod, you don't even have to add an aux-in like I did, you just move it.