[cmb]

I determined that the hiss/static is coming from all four speakers, so either the amps are working fine or they're all degraded in a very similar way. The static is always a constant low volume, it's present whenever the stereo is on (regardless of whether I'm playing the radio, CD, or aux in), and it's unaffected by the volume, balance, or equalizer controls. Fortunately, it's such a low volume noise that I doubt I could even notice it with the engine running or with music playing.

I'm thinking that this CDM aux input mod must not have caused this problem. Since the aux audio seems to be injected downstream of the signal from the head unit (i.e. the head unit audio is switched out and the aux jack is switched in), I would expect the nature of the static to change when switching between the two if it were caused by this modification. Since it doesn't change, it seems like this caused by something else...

Fortunately, the quality of the audio from aux input is quite good -- basically indistinguishable from CD audio from the head unit.

EDIT: I added a note about the cut traces in my post with the photos of the mod.