What I could find was either 86280-0W420 or 86280-OW150
A lot of talk about replacing the amplifier in threads in here though. The dealership probably won't have it.
here is a replacement thread.
4th Gen 4Runner Subwoofer and Stereo
The subwoofer box is looking expensive. You may be reaching the point of no return financially on this system.
Plenty of amplifiers on ebay you just have to find the perfect part # which is what you are trying to do.
Rear base mount.
2003-2005 Toyota 4Runner Rear Speaker 86150-0W040 | ToToyotaParts
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