My WRX is the first car I've ever been in with an auto-dimming compass mirror. Don't get me wrong, I love it, but for my own personal tastes, it seems to dim a little *too* much sometimes, almost becoming deep purplish - so much so that I can see the headlights of the cars behind me, but it becomes difficult to actually see the cars themselves (obviously worse when it's really dark out). So I tried this out and so far it seems to be working pretty well. The mirror still gets dark in response to a bright light, but not so much so that I can't see behind me anymore.
This has got to be one of the silliest "mods" I've ever done, but it works, and seems to work well, so I guess I shouldn't complain, but when you see the pic I think you'll understand what I mean (it's scotch tape)
Obviously you'll have to try it and see for yourselves if it works. I only used one layer of tape.