Horn replacement
By Victor Nomura (vnomura), November 14, 2000

How to change the horn in your Celica to make it sound better or louder. Maybe add an air horn???

Please follow the instructions on how to remove your front bumper first to get access to the 2 horns in the front.

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12 volts is applied to horns to make them sound. Please use a horn rated at 12 volts to make it work. There are so many different types of horns out in the market. You will have to be creative in connecting the new horn's wiring to the stock Celica's wiring. Please test and honk the horn before you put your front bumper back on.

Source...
http://www.celica-garage.cityslide.com/articles/article.cfm/522/159