I added the rear speakers via an OEM kit around 20 years ago to my MY97. My car only had the front speakers so this was the biggest improvement in cabin music quality by a factor of 1000%.
Anyway, over the years the speaker foams have deteriorated on all 4 rears and they started crackling when turned up. So it's time to replace them.
Of note:
1. The OEM speakers were 6 ohms each, 2 pairs of 2 in parallel - making each side 3 ohms. Not the 8 ohm to 4 ohm arrangement I had assumed.
2. I bought Ayima branded speakers from Aliexpress, 3 inch, 8 ohm, with square bezel. $100 AUD for 4.
3. One "tab" of the square bezel needed to be bent to fit the shelf housing - this will be obvious when you try fitting it.
4. One speaker from each side needed to be lifted by around 5mm to clear the shelf fitment tubing. I achieved this with washer paddings.
FYI. FWIW.