First, remove the trunk lid release/fog lamp switch panel.  A small flat blade screwdriver can be placed under the switch plate.  Pry up gently and the switch plate will fall out.
 

The switch plate, removed from the dash..
 

Once the plate is out of the way, you can see a torx screw.
 

Remove the torx screw.
 

And, disconnect the wiring harness from the trunk release/fog lamp switch.
 

Then, move your hand down along the knee plate and you will find another torx screw.  Go ahead and remove it.
 

Next, move your hand to the right, along the knee plate and you will find another torx screw.  Remove it.
 

Now, look up and identify the air temperature sensor plate.  Remove it with a flat blade screwdriver.
 

If you have automatic Dual Zone Air Conditioning, this exposes the air temperature sensor and another torx screw.  Be very careful not to touch the exposed air temperature sensor.  It is vulnerable with the vent cover removed.
 

Remove the last screw.
 

Next, begin to pull the knee plate away from the dash with your hand.  A gentle pull should release the panel with a snap.
 

Slide your hand up and along the panel.  This will cause the upper part of the knee plate to separate from the dash with a snap.
 

This next step can be complicated.  The lower knee plate panel has a tab which is inserted into the dash at the factory.  It is under the console panel which extends over the ignition switch and air temperature sensor.  You must pull the plastic console panel away from the knee plate very gently, as to prevent from cracking it.
 

By positioning your left hand under the end of the console panel, and by pulling the knee panel down and away with your right hand, you shouldn't have too much trouble getting the knee panel out from under the console panel.  This technique requires the most skill, as you must be careful not to crack the console panel.
 

Once you get the knee plate away from the dash, remove the air temperature sensor (if you have Dual Zone A/C installed in your C5.  In 1997-2000 years, squeeze the two plastic retainers on the rear of the sensor and then push it through the knee plate.  In 2001 and up years, the temperature sensor twists off.
 

Then, remove the connector from the sensor.  The connector retainer is rubber-like and can be pulled away from the connector with a fingernail or small flat blade screwdriver, allowing for the connector to slide out.
 

Put the sensor and the knee plate aside.  The hardest part is complete.  You're halfway there!