I had to replace one of the side mirror assemblies on my 2005 tC, and searched the forums for how to do it. There are posts that say how to do it, but nothing too in depth. It's not that complicated, but I'll give a step-by-step of how I did it anyway.
On the inside of the door there is a triangular plastic cover opposite of where the mirror attached. Pull/pry that off (it's just clipped in). That will reveal the bolts and plug that connect the mirror.
Unscrew the bolts and unclip the plug. On the outside, there are two little clips that attach the assembly to the rubber seal on the edge of the door. Unclip those, then take off the old mirror assembly.
Now you basically put on the new mirror assembly by going in reverse order: attach the seal clips to the mirror, slide the bolts through the three holes and tighten them, and clip the power plug. Obviously you pop that triangular cover piece back when that's all done. That's it.
Again, very simple and certainly something you don't want to pay a dealer's labor rates for. It takes 5-10 minutes at most.
I'll also mention that I got mine from trdsparks.com for about $145. I recommend them.
Hey Lucas, or anyone else who successfully removed/installed their side mirror assembly:
Thanks for the step-by-step. Seems easy enough, but I pried the triangular piece off only to discover that I am unable to unscrew the bolts. I don't think I have the right tool; what tool/gauge did you use?