installing a horn button

You shouldn't have to change anything under the hood. Just locate the horn wire at the base of the steering column and run it through a switch to ground.

See the wiring diagram below for the color of the wire you need to run to the new button

I recently purchased a 1999 toyota camry only to find out that the horn was not working. When pressed down the horn emmits no sound. I checked the fuse and it is in good shape. I also checked the horn itself and that too is in good condition. I read online that there maybe something restricting contact from the horn button (external) to the horn button (internal; steering wheel). My question is how I would go about removing the horn panel on the steering wheel without affecting the airbag. Also, could it be something else other than the horn itself? Thanks

You said the horn 10 Amp fuse is okay and the horn is okay. Try checking the Horn Relay. On the fuse/relay box beside the left fender, The horn relay is the black rectangle relay (beside the green square relay). It should show "HORN" at the back of the fuse/relay cover.

(Sorry, I do not know how to remove the steering wheel.)