I agree the belt chime is completely stupid. I will tell you what I have figured out so far and as soon as the weather warms up I will figure out the rest,
There is a magnet inside the seat belt base (the female part), when you insert the seat belt clip (the male part) it pushes a magnet down inside the base, that magnet hits a sensor on a small circuit board, once that contact is made, the car thinks the belt is buckled.
I put a magnet on the sensor thinking I could just fool the system into thinking the belt was always on. This did work, but there was a complication (I should note that I was only fiddling with the passenger side) and that is the air bag sensor.
For some reason when you put the magnet in place to fool the sensor, after a few days it will start turning the air bag off when someone actually sits in the seat (first day it works correctly). If you remove the magnet and put it back, the air bag will turn on again (like it should) when someone sits in the seat. After a few days it goes back to turning the airbag off.
I pulled the seat belt BEHIND the pasenger seat and plugged it in to fool the system, I get the same result as the above with the magnet in place. For some reason it starts to turn the air bag off after a few days. So the magnet theory is good, but the circuit must have some kind of timer on it that screws with the airbag if the magnet is in place for say 48-72 hours without the connection being broken.
I hope that made sense.
What I have concluded is that the speaker that makes the noise is what has to be disabled so you don't screw up the air bag system. I read somewhere (maybe in this forum somewhere) that the speaker that makes the chime is securely connected into the side of the instrument cluster (and the connections are very hard to get to).
When I get some time and it warms up, I am going to try and find this speaker and put an on/off toggle switch in line with it. This way I can turn on the chime for the state safety inspection (and if I ever sell it, I can solder the wire on and take the switch out easily) then all other times I can turn it off so the car 'feels' more like my trusty 1995 camry
I also saw somewhere instructions on how to turn the gauge cluster lights blue, this had a detailed explanation on how to remove the gauge cluster. I figured that would be useful for finding this speaker. If I can dig these links up again I will post them. This spring I plan on taking pics and writing up how to disable this blasted chime, it does seem to annoy a lot of people, but few post about it due to the on going seat belt lectures that prevail. I have files a complaint with toyota on it as well.
For those of you that have nothing to add other then 'wear your seatbelt' please don't post here, no one wants to hear it. Start your own thread about the joyous wonders of seat belt wearing if you would like, I promise I won't post there.
Meanwhile I will be doing 0-160mph runs on my seatbeltLESS motorcycle through your neighborhood