Just shine a light into your doors lower speaker grills and take a look at the condition of your speakers. In my experience, most R35 GTR’s have mould on their speakers. When mouldy, they won’t be performing as they should as the cone will be flexing as it moves. Eventually, they will get so bad that you will hear strange noises from the speaker and then they will fail completely.
They go mouldy due to water that comes in from your door mirror area.
As a better alternative to replacing them with the same Nissan rubbish at their inflated prices, I have a solution using Audison’s water repellent treated 6.5″ speakers.
I’ve put together a fitting kit containing all your need to fit these yourself. You get the pair of speakers, sealed GTR spacers (ABS material), wiring loom and instructions on how to remove your door panel and fit them.
The kit fits both Bose and non-Bose equipped cars from 2008-2020