There are three pedal switches on the 911 Carrera, two on the clutch pedal and one on the brakes. They get a lot of use, and they all have a tendency to fail once and a while. Replacement is really easy, and should take only about 30 minutes.
For the brake switch, the first step is to verify that your brake lamps are not working. If you're by yourself, then wait until nighttime and back up against a wall. You should see the reflection off of the wall when you step on the brakes. If you don't, then your switch is probably shot. It's important to note that the switch has redundancy built in, and the warning system should tell you when your switch is failing, but this doesn't always work.
The clutch pedal has two switches. One switch controls the cruise control (turns it off when you push in the clutch), and the other won't let you start the car without the clutch pedal pushed in. The micro-switch only activates when the clutch is fully depressed where as the cruise control switch disengages the cruise control as soon as you begin to activate the clutch. If you're having intermittent starting problems with your car, I would look at the micro-switch first.
Curious, but the cruise control switch is identical to the brake pedal switch except that the cruise control switch is red, and the brake pedal switch is a translucent brown. Both switches have three of the same the same part numbers printed on them.
The switches are relatively easy to access, as they are located right above the pedals. You will have to open the passenger door and then stick your head into the footwell to see them. You may find removing the knee panel gives you more room. Replacement is easy--simply unplug the harness from the end and then twist to remove the switch. If you need to manhandle and/or break the switch to remove it, that's perfectly okay, since you will be replacing it with a new one anyways. The clutch pedal switch that prevents the car from starting is a standard micro-switch mounted with screws.
To finish the job, simply install the new switch into place, and reconnect the wire harness. Test the brake lights to make sure that they are working properly, and then reinstall the knee panel.
Here are the two clutch pedal switches. The yellow arrow points to the micro-switch that prevents the car from starting if you don't have the pedal pushed in. The red switch is the one that deactivates the cruise control when you push the clutch in.
Here's a photo of the brake pedal switch (green arrow). Simply unplug the harness, twist the switch, and then remove it from the chassis. The lower left inset photo shows the brake and cruise control switches together. The only difference between these two is the color of the housing.