I have a 1994 cavalier it doesn have rear defroster and I want to ad one..what parts do I need and is the wiring already there for it or do I have to add it too?

You'll need a rear window with the defroster and the HVAC controls with the rear defrost button.
The wiring might already be installed on your car. You can remove the HVAC controls to see if there is a pigtail for the button and look around in the trunk or under the package tray to see if the connector for the defroster are there. If they aren't, you can get them off the car you get the window and controls from. Older GM cars often had the wiring for most options in the harness, even if the car wasn't equipped with them. They would just be tied off somewhere in the wiring, or would at least have the wiring connectors there. I'm almost certain my 92 convertible has the connector for button on the HVAC for a heated back glass, even though it didn't have the heated glass or the button for it on the controls. But I don't recall seeing the connectors for the glass, but I won't say they are not in the car somewhere.

Just to be safe, I'd get everything off the other car to make sure you have all the wires/connectors to hook everything up. Take pictures or draw a wiring diagram of anything you remove off the donor car.