Left speakers weird sound issue. Might be a quick fix?

So I upgraded my 2011 Mazda 3 (Non-Bose) door speakers with Pioneer TSG6845Rs. The issue is that only the front right and rear right sound fine, but the 2 left speakers have faded and have lost about 80% of their volume.

With the stock Mazda head unit, the issue started with the new speakers. I replaced the head unit with a JVC-KD-R980BTS and the issue still ensued. The weird part is when I connect my home theater amp to the speakers and pump the volume way up, they burst back to life! Then they play perfectly fine for a few weeks, but the same problem comes back, and the left side begins to slowly die out. Of course I could pump them up every few weeks, but that's just a nuisance //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif The sound is low and "fuzzy" like coil rub.

When I put the front window all the way down I hear the window tap and glide over the speakers. I have a feeling it has something to do with the electromagnetic properties of the door, or the mounting space of speaker, but that doesn't explain why the rear does it too.

Any pros that might know a solution? I've browsed many forums but never found a fix.