Car Speaker Question - Crackling Sound

[aphex]

There is a crackling sound coming from my rear left speaker. Sometimes its loud, sometimes fairly soft. Its not the sound of a blown speaker, more like interference or a loose connection, but it crackles even when im not moving, its not speed/movement related.

Unplugged the RCA's at the back of the HU (still crackles).
Unplugged the RCA's at the amp in the trunk (still crackles).
Unplugged the Speaker wire at the amp, still crackles.

Any idea how i can diagnose this?