I have a 99 Deville and I want to upgrade to a CD. What would be the best way to go about it? Is a aftermarket doable or how about a donor car? Will I have to worry about the theft programming? Any and all suggestions?

Ok, I'm not totally sure, but I think the base 99's still had the changer in the trunk. Does the radio have a 'Source' button? If so, the radio can have a changer added. If not, you have a very 'base' radio, and it cannot have the changer added. The Concours had the changer in the console, or the glovebox, I'm not sure?

The changers I've messed with, do not have the security stuff that the radio has. Just plug it in and if the radio will support a changer, it will recognize it and you are fine.

Take the spare out of the trunk. Pull up the carpet from the right side, clear down to the bottom of the spare area may be necessary. You should be able to find a wire with a what, a 9 pin connector, either down in the spare area, or over to the right side behind the trunk hinge. The stock CD changer, a 12 cd changer, was bolted down in the right rear area of the trunk with a bracket. I've run one with just velcro on the bottom glued to the carpet to hold it in place, no bracket.

Be sure you find one that fits a 99. There are several that look pretty much the same on the outside, but will only work for certain years.

Ebay is somewhere to look for these. There are also internet companies that sell new ones, for about $350. There is also a company, USA Spec, that sells versions for each different version of Cadillac needs, for about $250.

So you are saying all I have to do is install a CD changer from another car and my tape deck radio will play the CDs? Mine does have the source button too.

So you are saying all I have to do is install a CD changer from another car and my tape deck radio will play the CDs? Mine does have the source button too.

Yes.
Go hunt for the wiring in the trunk.

I will look for the wires as soon as the weather breaks!

you should have th plug back there if your hu says source on it

you cannot add a factory in dash cd player to your car unless you have bose as they all rely on an external amp

CadillacSTS2003, Do you agree that I can just add the changer in the back and I will be able to play CDs through my Non CD radio without the amplifier?

oh yea
the cd changer mounted in the trunk is whats known as a slave unit
ie it relies on the main hu to control it
your car should be prewired
the plug should be on the pass side behind the trunk mat close to the power antenna mass