The kid who owned my 98 LS before me broke the passenger-side dash vent out. He said he was trying to figure out how to put Tweeters in them.
I found one 7 mm screw low and behind after removing the glove-box, and 2 clips on the vent top. But it still isn't budging??? Thanks!

 

I think you are going to have to rotate the entire dash forward to allow access to the rear part of the vent in order to get old out and new one in.

I think the duct work is in the way otherwise.

Do a search on heater core replacementing...you might be able to view some pictures of the back side of the dash to get a better idea of the retainers involved.

Rotating the entire dash is about 30 - 45 minutes of removing the various trim panels and such. Not real bad...not something I would want to do in cold weather though.