I have noticed over the last few years on Audi vehicles that sometimes
they will have a red taillight on the left side that shines brighter
than the normal running taillights, almost like an error message.  The
light in question is always to the inside on the left side in the
rear.  I have seen this on many different models (especially A6-A8's)
that are anywhere from 15 years old to almost brand new.  What does
the light mean?  Is it some sort of diagnostic code?  Please help
solve this mystery that is bugging me.

Hello rennaisanzmn, when I read your question I thought it sounded
strange. I believe that I have found the answer.

http://groups-beta.google.com/group/rec.autos.makers.vw.watercooled/browse_thread/thread/9496544ca56d569f/f060f4061455a7e9?q=Audi+Tail+light+left+brighter&_done=%2Fgroups%3Fhl%3Den%26q%3DAudi+Tail+light+left+brighter%26qt_s%3DSearch+Groups%26&_doneTitle=Back+to+Search&&d#f060f4061455a7e9

Last weekend I checked out an Audi A4 Avant wagon. 
The dealer said that there is a REAR fog light 
(red, left rear only) on the Audi.  It is allegedly 
much brighter than the standard tail lights. 


After reading this I searched further and found this:

http://www.audiworld.com/model/a8/00/specs.shtml

Rear fog light (integrated in left rear light cluster)

http://www.thepartsbin.com/repsite/audi~tail_light_assembly~reparts.html

http://www.audiworld.com/tech/ext20.shtml

Rear Fog Light Modification 
This is a good picture of exactly what you describe.

Hope this solves your mystery, Redhoss

Great!  Not as "mysterious" as I hoped the answer would be, what an
odd feature, I thought it was a defect.  One less thing to wonder
about, although now I wonder if it makes a difference in fog... Oh
well, another question another day.  Thank you!