Speaker problems - how can I fix this?

I have a 2010 Toyota Gen 3 Prius. I have a problem with my speakers, and so I took apart my whole radio and such, but am now wondering which cable should be changed/should the whole thing be changed?
When all the wires are plugged in, my front speakers sound fine, but the back ones have A LOT of static.
When taking apart my radio, I noticed the nundle of wires coming out of the 10 pin connector were taped together (it was clear it was taped together to fix it and not like that originally). The tape was coming loose, and when I would hold the wires steady in the taped position, the static would stop. I therefore took out the tape that was coming apart and tried to tape in the same way with electrical tape (see red tape in pictures) but the problem wasn't solved.

I wanted to just change the wire bundle that was taped, but I have no idea if that's even doable and how far back into the dash these wires go. I saw wire harnesses on sale for relatively cheap but before buying something that might not even be the solution I thought I'd check here.

I also read some places that it might be a fuse in this wire; if so how can I tell?

So my question is has someone had this before? How did you fix it?
How can I diagnose what the problem is?
How complex would it be to change the wire bundle that's taped in the pic?

The red tape is the tape I replaced at the exact same place where there was the initial tape I found, which was coming undone. The 10 pin bundle is the bundle that was taped. According to me, I believe the problem stems from that bundle and/or the copper colored wire, as these are the only two that 'fix' the static when moved around.