I kept the cat off my stock exhaust just in case - need to make a 'test pipe' adapter for it, although I thought the '99 shouldn't require it.
If it had a cat from the factory, it needs it. As I understand it, some '99 did, some didn't. There should be a sticker under the hood, usually on the radiator support, and it will say Catalyst or Non-Catalyst.
Seems to me there would be a black market for new stickers.