I thought I would share this, when I owned my 95 Monte, there was a piece of plastic going between the bumper and core support that acted as a debris shield and directed the air to the radiator. According to GM, it also helped with aerodynamics. Well, when I got my 99 this piece was missing and I couldn't find a new one from anyone, body shops, gm, nobody. So I decided to make my own. I used corrugated plastic because it's cheap, not to proud of the outcome, but it's a base line.
Lemme know what you think.

 

I can't remember if my 99 has it yet or not, I owned a 96 Monte I believe it had one on it. I have the RK sport Ground effects on the 99 and they do a great job and look real good.