Here are all the bulb replacement numbers for the C5. The first one is the one you need. Also, I have posted the corresponding Sylvania bulb replacement part numbers.
Parking light
4157NALL
4157AST Silverstar Signal Lighting: Complete the Look: Crisp, Clean, Style
Front turn signal
4157NALL
4157AST Silverstar Signal Lighting: Complete the Look: Crisp, Clean, Style
Rear turn signal
3057
3057LL Long Life Upgrade: Up to twice the life of the standard lamp
Tail light
3057
3057LL Long Life Upgrade: Up to twice the life of the standard lamp
Stop light
3057
3057LL Long Life Upgrade: Up to twice the life of the standard lamp
High mount stop light
Assembly
Fog/Driving light
893
893ST Silverstar High Performance Lighting: The Whiter and Brighter Halogen
License plate
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Back up light
2057
2057LL Long Life Upgrade: Up to twice the life of the standard lamp
Front sidemarker
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Rear sidemarker
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Glove box
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Map light
212-2
Step/Courtesy light
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
37
PC168
Under hood light
211-2
211-2LL Long Life Upgrade: Up to twice the life of the standard lamp
Instrument-general
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Automatic transmission indicator
194
194LL Long Life Upgrade: Up to twice the life of the standard lamp
Here is a short video which shows how I replace the daytime running light bulb.
YouTube - Changing Your DRL's in 2 Minutes!