If your car did not come with one of these and you don't expect to change the stock exhaust and you'd like to surprise a passenger or two... you should add one. It makes a surprising increase in full throat acceleration sound inside the car (especially when you're in traffic to reflect sound back to your car)- does not change the level outside, but you can't tell inside the car... it just sounds good (and mean!).
Check the back firewall for an opening under the black felt in an almost straight line back from the spark plugs (driver's side). I tried to add a couple pics to help locate it, but my desktop has updated and now I can't do shite! If you've got one and Chevy has simply plugged your intake opening (as in FarmerFran's car), I'm betting it is cheaper to plug and just cover the hole than doing away with it.
I'll try again with the pics- sorry.
BTW- this is not just a hose to carry sound into the cabin. It's got diaphragms and other components to protect you from fumes/ carbon monoxide, etc...