The factory key switch is on the end of the drivers door, if you don't have the factory alarm there's a plastic plug like this (on the left)
From what I remember, the factory alarm horn sits in the "smugglers box" just above the A/C stuff.
Here's some info on bypassing the stock system
http://forums.pelicanparts.com/porsche-911-technical-forum/766645-911-sc-alarm-bypass.html