You need a module for aftermarket radio.
the reason is simple: there is no accessory wire!
The original radio is what make the "annoying" but such usefull chimes (door, key, light, ICP lights, ....)
It work by having a serial data wire from the BCM to the radio. That is how RAP also work.
So, the module have the chime in it, decode the serial data, provide the needed accessory wire and also send a Class2 healthy code to the serial bus.
Such modules are easy to find (when you know where to look and for what terms). Even walmart have some! (however, I have issues with mine, when it's bellow -15C the engine is hard to start, and the battery have some issues maintaining a good voltage. the result: the module don't turn on the radio. I have to let the engine run a bit, shut it down, open door, close, restart engine and it usually work), hence why I suggest a quality one instead of the cheapie one.
They call it a GMCO, or gm chime retention module. Get one with a genuide AC Delco chip, that is made for your car brand, model and year (and trim, however it should be universal). They are around 100$
Alternativelly, you can find a switched accessory wire somewhere under the dash, good luck!
As for the factory radio code.... search the forum, there was someone that posted how to get the code in the past...