Yep. It's an "udder". There are three of them on a C5, all on the driver's side on the firewall under the master cylinder. Notice that the bottom is split, allowing water to drain through. Whenever you wash your car, make it a habit to reach under there and feel around for the three of them (two are in front, and one is behind the others). Milk them a little and you'll probably feel pine needles, leaves and just plain muck work its way out of each. If they clog up, it can cause trouble. Good luck!
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