The 997 is like the 996 in many ways (as it was just a very deep facelft).
Pictures to illustrate mounting location (no need to remove PU, unless you want to), you can access from underneath.
The horns are conveniently located to collect water, corrode and die.
Just to the right of the left rad (behind rubber air channels):
The horn placement is as shown in the pics, it is held on by one 13mm bolt (and obviously a plugged electrical connection).
You can access it by removing the bumper, or from underneath if you can lift the front high enough for access.
Assuming you don't access via PU as photos, here is the alternative:
Required Tools:
13 mm socket (small handle)
Star T30 screwdriver
Ramps or jacks and axle stands
Procedure:
1) Raise car on ramps, or just driver's side front with a jack (axle stands for safety).
2) Using the Star T30 screwdriver remove the six centre screws on the underbody panel that attaches to the lower front PU.
3) Pull the the lower front bumper down, feel for the horn and using the 13mm socket inside the bumper, loosen and unscrew the retaining bolt on the top of the horn.
4) Pull the horn unit down until you can access the electrical connections. (there is a small release tab on the connection to release).
5) Connect new horn and refit as reverse of removal.