I have an OEM auto-dimming mirror with compass. It came out of my 2003 WRX, but should fit any WRX up to 2011.
Unfortunately, the auto-dimming function no longer works, so replaced the mirror with a new one from the dealer. There is no clouding or any other defects, and the compass still works great. In fact, you only notice any issues at all at night.
I'm asking $40 shipped, the new cost me $165. All hardware and installation instructions included.