It was pointed out to me months ago that my driver side mirror turn signal was not working. No CEL (check engine light) comes on the dash with this issue because they are not required by law to be there. If you do have a VCDS though you can pull the code which will look something like this:
00109 – Left Mirror Turn Signal Lamp (L131)
012 – Electrical Fault in Circuit
You can not just replace a bulb in these because they are LED’s. So you must replace the whole signal unit.
The cheapest place I found for just the driver side mirror turn signal was http://www.pelicanparts.com. Part number is 1K0-949-101-M409. Other sites had some smoked turn signal sets but they were either too dark or looked cheap. I could of also purchased a whole new set, but my passenger side was still in good condition.
I did have some issues with getting the housing off, instead I just worked around it to get the signal off and install the new one.