With reduced integrity the chance of data loss or corruption during transmission increases. There are typically low level integrity checks and forward error correction used to compensate for some of that on serial links, but they can only do so much. If data is detected lost its pretty much up to the upper level protocol on what to do next: One could retransmit, do nothing, or just use the corrupted data. Since retransmission would not help audio protocols they likely will use the other options, and so you get degraded sound.
This can cause resistance pockets in the wire itself especially when since it's probably continuously moved around every time you plug in the phone, further stressing the wires.
Lastly condensation inside the car can corrode contacts slowly causing further signal degradation.
Your best bet would be a thicker gauge wire usb cable with gold contacts.
The gold is added by the manufacturer to prevent corrosion of the contacts during transport and storage of the cable/device prior to first use.
Contacts corrode on exposure to oxidants. They can be re-made by disconnecting and reconnecting, which (if the contacts are properly designed) wipes their surfaces.
I have never heard of "resistance pockets" in wire, but I suppose that could be one way of describing broken conductors inside the cable that only continue to work because the end pieces are still making contact. Until the ends corrode or the wire is flexed in a way that pulls the ends apart.
It doesn't kill the connection, but audio quality goes down very noticeably. Replacing it with a new one seems to fix it for the next half-year. Is this real? Whats the mechanism behind it?