So you have decided to remove the factory radio and replace it with an aftermarket head unit.  To do so, you will have to remove the Instrument Panel (IP) Cluster Trim Plate.  (You would think they could have come up with a simpler name.)

This article was done with the help of the Helm Impala service manual.  I highly recommend that you should pick one up.  At $120, it's not cheap.  However, it beats any manual you can get at an auto parts store.  If you are interested, the contact information is in the back of your owner's manual or go to the Accessories page.

Apply the Parking Brake.  Using a small flat-bladed tool, remove the ignition switch bezel.  I am using a paint lid remover that you can get at any hardware store or paint shop for about $1.  It's like a screwdriver with the end bent 90 degrees.  The other end is a bottle opener, making it one of the most useful tools in my toolbox.   Pry around the edge of the bezel and pull straight out.  If you have a column shift, insert the key, unlock the cylinder and shift into "1."  You might have to step on the brake to move the lever.  I think there is an interlock, but I have a LS so it might be different.
Remove the left and right fuse box access covers.  Remove the screw as shown from each side.  You will need a 7mm socket or a Phillips screwdriver.  I recommend the 7mm socket.  IMPORTANT: I believe all the screws were the same length, but make sure you compare them as you remove each one.  You may want to keep them identified for easy reinstallation.
Ok, the next couple of steps are not mentioned in the service manual.  Remove the left Close Out Panel.  Press the two tabs down and it will fall out.
Remove the two screws holding in the Steering Column Filler Panel.
Starting from the left side, pull the Steering Column Filler Panel straight out, toward the rear of the car.  It uses press-fit fasteners.  Be gentle.  If it doesn't feel like it's going to move, take a look at the back of it from the area that was covered by the fuse box access panel.  You should be able to see the fasteners.  After you get this panel out of the way, remove the indicated screw.  Be aware that the trunk release switch is still plugged in.  You can leave it connected, just don't strain the connector.
Remove these two screws.  This is where the socket is better than the screwdriver.
This is the is the fun/spooky part.  First check to see that you removed all five screws from the IP Cluster Trim Plate.  (Seven, if you include the ones from the Steering Column Filler Panel.)  Starting at the right end of the Trim Plate, grasp it, and pull it rearward.  Work your way to the left.
Disconnect the connectors from the hazard light switch and the traction control switch (if you have one).
Gently guide the IP Trim Plate out.  You might have to move the steering column to better clearance.  Be aware that the clips that held the trim plate in place are sharp and can scratch/cut things.  Just work slowly.  This is what the trim plate looks like from the back.
Here is what you are left with in the car.
Remove these three screws and you can pull out the radio.  Disconnect the radio wiring harness and the antenna cable.  WARNING!!!  The service manual isn't really clear on this point, but if you remove the radio, you may have to get the dealer to use a scan tool to set up the radio.  Keep this in mind if you intend to reinstall your radio.

  I did a little more reading and this is what I found:

"The 'Theftlock' theft deterrent system is a feature on all base and uplevel radios.  There is no need to program a security code into the radio as in past model years.  The theft deterrent system now utilizes class 2 serial data to determine if the radio is in the appropriate vehicle.  Each time the radio receives the 'run' power mode message, it compares the VIN information it has stored to the VIN information received from a module on the class 2 serial data circuit responsible for transmitting that information.  If a mis-match occurs, the radio display will indicate to the user the radio is locked. Once this takes place, a DTC {Diagnostic Trouble Codes - Nathan} will set, the radio will not respond to any button presses and become inoperative.  The two scenarios able to cause this condition are:

1. A radio is installed from another vehicle

2. A module which communicates on the class 2 serial data circuit which supplies VIN information to the radio is replaced and not properly setup with the correct VIN for that vehicle.

The only way to unlock the radio is by using a scan tool."

The way I understand this, as long as you do not switch radios from car to car, unplugging the radio will not automatically lock it.  So unplugging the radio and relocating it to the trunk using an aftermarket harness should not require that the radio be reprogrammed with a scan tool.