Can I replace the factory (am-fm,tape) with a cd unit from a newer model cadillac? If so, what amount of modification is involved.

Thanks for any help.

You could, but it would be paying an excessive amount and getting an inferior product. All head units are not equal, the most important factor tends to be the wattage they put out, OEM or Stock head units are underpowered! You will save tons of money and get a better product shopping at a local car audio store or www.crutchfield.com

Crutchfield also has great tech support and you can call them up and ask them to explain anything regardless if you have bought something from them yet or not!

You could find a Cadillac with a Factory Delco CD Player in the same model car at a junk yard and have it installed or you could buy an aftermarket deck and a "Universal install kit" and have it installed or you can go the cheap route and buy a Cassette/CD Adapter that looks like a cassette with a patch cord on it that plugs into a "Portable" CD Player.
http://www.carstereohelp.net/wireharness.htm

I just had installed a 12-cd changer in the trunk of my Deville and it is sweet! It'll run you about $350 for the package. Mike