I'm in the process of swapping everything over to my new cutlass body & the new car does not have the rear defrost. The old cutlass has the rear defrost on it. I know I can swap the rear window over but how can I make it so the defrost is functional.

 

Basically, you will need to swap over the window, wiring, switch, and any associated parts.
Look in your service manual at the defroster wiring diagram and you will see the parts that need to get swapped.
The dash will need to drilled for the switch if it does not have the hole.
If this is a true body swap, then it should swap over pretty easily.
Also be sure the alternator is correct for a car having defrost (not sure if there is a difference then or not).