[MJ777LR]

ctuna, thanks for the input. I'm also not too keen on putting in an aftermarket HU. I like the stock look and how the OEM HU blends into the interior.

So, just some updates on this after troubleshooting over the past week... I received the Metra harness and modified it so that I could connect my external mp3 player to the harness using a 3.5mm jack, in place of the OEM HU. So basically, I had this setup going:

External mp3 player -> Metra Harness -> Factory Wiring -> XD600 amp

Result: No noise/hiss whatsover - Everything was blissfully silent!

So, would it be fair to conclude any of the following from this experiment:

1) The factory wiring from the HU to the amp is not picking up interference and is not the cause of the noise.

2) If (1) is true, then looking at the next component up the line (the OEM HU), the factory grounding of the HU could perhaps be leading to differing 'ground potentials' between the HU and amp. If so, perhaps routing a dedicated ground wire from the HU to same location where the amp is grounded could fix the noise?

3) If (2) does not fix noise, could this be a bad HU, perhaps a unit that's not properly grounded internally?

Some other observations:

1) When OEM HU is off, there is no noise/hiss. When OEM HU is on but muted (volume set to 0), noise/hiss is also not present.
2) Turning volume to 1 is enough to bring back the hiss.
3) Hiss doesn't seem to get louder when the volume is increased.
4) Hiss doesn't appear to change with rpm.

If anyone can deduce anything from the contrary, please do chime in, as I would really like to get your opinions. Thanks all!