Located on the passenger side in the engine compartment you have your main air conditioning lines. Each of the main lines has a capped valve (fill point), that has a black plastic cap on the valve opening. (The one line goes to the Receiver/Drier and the other line goes to the condenser up front).
Both of these new chrome caps are made from copper. They have a small hole in the top similar to the original plastic cap it covers. A nice cosmetic touch even for those that may not like a lot of chrome under the hood.
Fits 1997-2011 C5 & C6