What is needed to change to heated seats?

Seats and controllers...

Its easier going after-market with this one. Adding factory is only possible on a Gen II and even then lots or wiring and since they can be prone to failure you may do all the work for nothing.

I plan on using it on my 97. I have the control switch I got from the junk yard, but the seats were gone. Besides the seats, what else is needed? Is the wiring already there?

The wiring probably isn't there and would probably cost a fortune from the stealership; that is, if the stock is not completely gone. Go aftermarket for lower price and availability. You can probably figure a way to rig the aftermarket to work with the Mark VIII stock switch to make it look stock...