Radio stopped working.

https://www.modifiedlife.com/2004-chevrolet-impala-car-radio-wire-guide/

You apparently did not hook the radio up to a hot wire when switch is on 'only'. The radio is constantly pulling power from battery even when parked and off with key out. Turn switch on, find a hot wire, turn key off. If power goes off, use that wire for the radio. That's it. Done. Stop replacing batteries now. As for the heater and defrost, you've given no symptoms other than the Battery drain.

did it just stop working? displays are a weak point on those gm radios ,, replace radio is your best fix,,, you MUST buy the wiring kit for it ,, gm uses a data cable that runs thru the radio and will not work if it is not routed thru an adapter . all kinds of strange things will then happen ..

I also have a 2004 Impala with the same problem. I searched the computer and found that a connection in the wiring under the carpet and behind the passenger seat gets corroded from the floor being wet. I'm in the process of removing seats and carpet to find this problem and hopefully fix it. I'm sure that your door chime quit working too.

Try this website. Thanks

www.radio-code.com

This may be a anti-theft issue. If it is and you want it fixed, the Chevy Dealter is probably the best place to have this work done, IF it can be done at all. My Chevy radio instructions tell me that if there is a genuine power loss to the radio when the anti theft system is activated (like someone breaking into the car and stealing it, cutting the wires) that the radio will not work again with out GM intervention.

If your radio looks to be working and no sound, open trunk and mounted underneath the deck lid you will see your amp or sound booster. It should have two plugs that plug into it. Unplug the one with two orange wires and with the car switch turned on, check to see if the two orange wires have power on them. If they don't get ready to take out the rear interior and underneath the rear carpet almost at the front seat you will see a big plastic sleeve full of wires on the passenger side. Inside this sleeve there will be one orange wire branching into two orange wires, check to see if it is corroded. That would be where I start.

if its a factory radio you must take it to a dealer 50 bucks to unlock it
its an anti theft device

It's a bad stepper motor (the device that operates the needle on the gauge).  Replacements can be found on eBay for approx. $4.95 each - just search for "stepper motor".  Just got 6, to replace all of mine.

Radios in the 2004 Impalas are tied to the vehicle's VIN through the computer, so it may be somewhat difficult to get the code. However, the following steps should work:

1. Press and hold the buttons for presets 1 & 4 (may be 2 & 3). The word "SEC" should show.
2. Press the "MIN" button. "000" should be displayed.
3. Press the "MIN" button again to have the second two numbers appear. Record these numbers.
4. Press the "HR" button for the first two numbers to appear. Record these numbers. -- Note: Your number sequence should be the numbers from the "HR" button followed by the numbers from the "MIN" button.
5. Call 1-800-537-5140. This will get you to the GM unlock code automated line. Ignore the prompt and enter "202108#". Next enter your code (remember, HR then MIN) followed by the * key. For example, 1234*. A 3 or 4 digit code will be given to you. If you get a code such as 0532, drop the zero when entering it into your radio.
6. Enter the code into the radio IAW the applicable manual.