I know there have been many posts on this topic. But i believe this to be worthy of its own thread as this is the legit 'factory' way of doing this without any wiring changes. This may work on other year models, but has only been confirmed on my 2013 TRD off road Taco.

First of all, the owners manual lists the seat belt alarm warning as 'customizable' indicating its possible to be normally turned off. it gives no instruction on how to do so, and presumably the dealer can do it for you. methods from earlier model years don't work.

i did not discover this solution, someone from a scion board posted these instructions which were originally from a prius. but they are confirmed working on the 2013 taco.

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Step one is buy a elm327 obd reader. mine was about 20 bucks, they are all over amazon. you can also use this in the future with your smartphone if you buy a bluetooth model to do all kinds of cool things.

2nd, plug it in, power up and get a bluetooth laptop/computer something to talk to it. do a normal bluetooth connect, password is 1234. open up hyperterminal, or putty or any basic terminal program. change the com port to your bluetooth serial (mine was com40 as it told me when my bluetooth connected). Connection set to 9600/N/8/1.

Then basically just plug in the commands below, and your seatbelt alarm will be permanently off. This is basically what the dealer will do if you bring it in, although they may have some fancy tool to do it from toyota.

Direct Connect CDT
>LM327 v1.5
?

>ath1
OK
* Turn on all headers so I can see it

>atl1
OK
* Turn on linefeed so I can read it

>at sp 6
OK
* Set the protocol to ISO 15765-4 CAN 11/500

>at dp
ISO 15765-4 (CAN 11/500)
* Display the protocol (make sure it took it)

>at caf1
OK
* Set auto formatting on so I can read it

>at sh 7c0
OK
* Set header to 7c0, that's where I'm going to send the command to

>21a7
7C8 03 61 A7 C0
* me - What's your seat belt setting?
* Taco - My seat belt setting is set to annoy you, C0.

>3ba700
7C8 03 7F 3B 78
7C8 02 7B A7
* me - don't annoy me anymore
* Taco - processing

>21a7
7C8 03 61 A7 00
* me - did you get that?
* Taco - yeah I turned it off, don't you see the 00?

Enjoy, please post and let others know if this works on other years. This is likely to work on any recent toyota or Scion vehicle, but no guarantees.