Where are the common places the C5 leaks? My 2000 sits in the driveway and when it rains both passenger and drivers side carpets get soaked just sitting there.
The windows seat well against the rubber moldings.
I haven't a clue where to start looking. Any suggestions greatly appreciated.

 

Check in front if the door. The weather stripping doesnt seal well and runs down the a pillar and slightly under the dash. Same thing happened to mine.

 

I would also check the udders. There is 3 on the drivers side and one in behind the battery..

 

Mine leaked and I checked my utters and they were all clogged. Hopefully this fixed my issue. Hopefully it fixes your issue as well.

 

There is a ton of information here on CF to assist. A lot of it is takes a hit-or-miss approach. Using the process of elimination will get you to a fix a lot faster.

The easiest first step would be to clean the udders as others have already stated.

Then, if you still have leaks, do a water test. To prep, use painter's tape to seal up all of the possible leak sources on the body (roof line, across the windshield, along the windshield pillars, etc.). Park your car, sit inside (a flashlight could be handy), and have a helper spray your car to simulate rainfall conditions. There should be no water coming in when all of the tape is in place. Then, have your helper remove one tape strip at a time (starting at the lowest or outer most points) and spray water again. Look around to determine, if possible, if and where the water is coming in. Remove the next piece of tape and repeat the drill.

Is the headliner (either side above windows) getting wet? Put your finger (or stuff a paper towel) in between the weather stripping and headliner. Could be a roof gutter leaking.

Is water dripping down the door pillar? It could be coming from the weather stripping piece above the door, at the front, or the one on the front top of the door.

Is water coming in from the firewall / above the clutch? As mentioned, clean the udders if not done so.

Here are some starting points for reading:

Leak Checks
General Water Leak Guide
FRC Leak - Can someone check my HW?
01 Z06 water leak - need advice

 

I've got a few Word documents that cover water leaks in the C5. If you'd like a copy of them, just PM me an e-mail address and I'll send them to you.