Does anyone happen to have the part number for a C5 A6 ('03 avant, 3.0), or now where to find such info??? I'm a little hesitant to order a new bulb as it might be the ballast/igniter as well. Any ideas on how to diagnose such an issue?

 

I guess the "easiest" way (and it's how we verify the cause at work) is swap the bulbs from side to side and see if the inop one is still the same side or is now the other side.