http://www.duramaxforum.com/forum/attachment.php?attachmentid=8152&stc=1&d=1254873916

Somebody whacked the driver's side mirror on my 07 2500HD.  I can find the
entire mirror assembly a couple places, but a buddy of mine had his 08
whacked this spring and he said the guy who whacked it was able to just
replace the single broken part, not the whole mirror assembly in his own RV
shop.

He had the same part(s) broken as I did.  The lower mirror and the plastic
backer that it mounts on.  I tried the same place my buddy said, but they
denied that they ever had any mirror parts or that they had any clue where
to get them.  Sigh.  I suspect the old guy was the one who hit my buddy's
truck mirror and didn't want to admit it in front of his old lady and kid
who were also there when I went in.  Maybe he really doesn't remember.

Anyway, can anybody point me towards getting just this replacement part.

http://www.duramaxforum.com/forum/attachment.php?attachmentid=8153&stc=1&d=1254873916

Stopping by a glass shop and having them cut a piece of mirror glass to fit
isn't going to cut it.  The plastic base is cracked.  My buddy said the guy
busted out the broken mirror the rest of the way, replaced the plastic part,
and sticky taped (the way it came) the new mirror into the new plastic part.

The local Chevy parts counter wants to sell me the entire mirror assembly.
They don't even acknowledge that there is a possibility the individual lower
mirror might be available.