Guys, thanks for the replies. I was aware that the radio was using
Class 2 databus to turn off and that there was no accessory wire that
turns off when the door opens.

But I heard that if I tap off the brown wire of the ignition harness
that it behaves like RAP.
Can anyone help me confirm this? I'm not great with locating things in
cars so I don't want to be splicing things useless and doing any
damage. I have the ALLDATA cd's for the 2000 cavalier but it's awful
and does not show the ignition harness and what the wires are.

Here is the result of some research I made which I stored in a .txt
file, but no concrete answers to my problem:

2 people have suggested the brown wire in the ignition harness,
apparently that wire stays on until the door opens but I want to
confirm this with some sort of manual.

I also found this circuit:
http://www.the12volt.com/relays/page5.asp#drto

---------------
Retained Accessory Power (RAP) Operation
Serial Data Control of Retained Accessory Power, RAP
The Radio receives a RAP message from the Body Control Module, BCM over
the serial data circuits. The BCM module monitors the ignition switch
position, battery condition and passenger compartment door status to
determine whether RAP should be initiated. The BCM module then sends a
message to the Radio telling it that it is in RAP power mode.

RAP will end when one of the following conditions are met:

The BCM module receives an input from the drivers door jam switch.
The BCM module receives a message from its internal timer indicating
the end of the RAP period after 10 minutes.
The BCM module detects a decrease in battery capacity below a
prescribed limit.
The transition from OFF to RUN/ON or ACC of the ignition switch.
The BCM module then sends a serial data message to the Radio ending the
RAP function.

Based on this you may want to find an ignition controlled line to tap
into versus the radio voltage.
---------
"In the left side of the dash, there is a fuse block.

There is a RAP Relay, or Retained Accessory Power relay.

It has a 30A circuit breaker - off the back of the fuse block there is
a yellow wire that feeds off this breaker.

This power source turns off after 10 minutes or when you open the
door."
---------
" After much research, I found that the only things truly powered by
the RAP circuit are the sunroof and power windows. I tapped into the
power window wires for 12V power. It's a 25A fuse and I'm sure it can
take another 1A maximum load. The radio has constant power. It turns
off by a serial data signal telling it do so, along with the HUD and
many other systems in the car. "
---------
"2000+ GM/Chevy vehicles with RDS capable radios do *not* have a
switched ignition power lead behind the radio.

Instead, the radio is hooked into the Class 2 Serial data line. When
the door opens, a signal gets sent that tells the radio to power down.
That's how it works.

Now, nearly all Chevy vehicles have an aux. power adapter somewhere in
the passenger cabin. On the Impala's, for example, it's under the dash
in front of the passenger seat. This adapter usually has four wires
going to it. Ground (black), hot (red or orange), switched (yellow),
and data (purple). The yellow wire is key controlled, and will lose
power when the ignition is turned to "off". It won't stay on until the
door opens.

No third parties yet make a device to really talk properly to the
serial line and thus be able to do things like provide a switched
power mechanism that works like the radio / door opening thing does.
So there's really no easy way to do what you want to do. But, it's
entirely possible for it to be done, it's just that there's no
equipment to do it."
---------
"You may want to check the fuse box. I know that a lot recent GM's
have a fuse in the box labeled RAP(Retained Accessory Power). I have
my SkyFi tapped on that fuse and it turns off when I open the door not
when the key comes out of the ignition."
---------
"The newer GM radios have a built in relay for switch (ingn. accy.
power) how it works I really don't know or care. Find the main harness
coming in from the car(firewall) and test the thick brown wire.
Usually theres only one thick one. Run your wire fuse it and it's
done. I have to do that for almost all new GM's when installing
radios."