For the sedan...it's the front bumper. The lower opening is wider on the 01-02. I've seen one put on a 01-02, and it just looks kinda jacked. There is a gap on either side of top of the WW front lip that lays on top of the bottom part of the stock bumper.

For the coupe...it's the front and back bumpers. The new front has those extra vent holes. And the rear has a little lip of it's own. But I know a guy on HS got the OEM coupe rear lip for 98-00 to fit on his 01-02 rear bumper.

Hope that makes sense.