The first mod I made to my 2008 FJ was to install the secondary 500Hz High Horn on the passenger's inside fender. Mountain drivers are all over the double yellow lines, especially on blind left hand corners, so a horn is something you get used to using a lot.
Unlike the 2007-2010 FJ models, there is no hidden secondary horn wire on the passenger's side fender wall.
You can still order the 2nd High 500Hz horn and 6mm bolt, and install it on the front passenger's side in the front (where it belongs), next to the stock Low 400Hz horn. There is already a 6mm threaded hole in the front.
I stripped off about 1/2" insulation on the gray wire going to the main horn (don't cut it), and soldered a 2' piece of 16 gauge wire to the gray wire. The other end of the wire I soldered a 1/4" connector, which connects to the 2nd horn. Wrap the connections in black electrical tape so they are water tight. The jumper wire was covered with black wire cable cover for protection and sealed at each end with tape, Tie wrap to the hood release plastic. Looks very stock. Will post photos if anyone is interested.
2nd 500Hz Hi Horn: 86510-60240
6mm Bolt: 90080-11545
2 feet 16 gauge jumper wire
1/4 wire connector
Flexible wire shield (optional)
Black Tie Wrap
The sound difference is amazing. It sounds like a big car now.