My 05 Monte has a factory AM/FM, cassette radio, I would like to replace it with a factory AM/FM cassette/CD unit. My question is will an unlocked unit still allow me to use the function programs in the radio, like turning off warning reminders and other settings. I hope this is clear.
Ebay has couple of nice unlocked units, plug & play.

 

You can use other methods to turn off the reminders, use the headlight and or gas pedal method. Give you a choice to use an aftermarket, can get better for less sometimes that route.

 

When looking for radios in the '00+ Montes, I am not sure there is a such a thing as "unlocked". I think no matter what it needs to be programmed for your car with a GM TechII scan tool.

I replaced my factory CD player for an MP3 factory CD player from a Venture Van/Cavalier (a radio GM says is "not supported" for my car, as a result I went through a fair amount of education). I had a guy dial it in with a GM Tech II and it works great.

FYI - Don't get hooked into the concept "You can unlock it by pushing these buttons and calling this GM number and getting an unlock code". That is valid for Pre-2000 Montes (the TheftLoc system was changed in 2000, TechII is the only way to unlock it).