I would be guessing...Submerse the housing in hot water? This might make the plastic more pliable. I was thinking of a way to mod my mirrors to do L.E.D. directionals and when I looked at the mirror assemble I didn't see a way to get the mirror out except maybe boil some water and dip the assembly in to make it give a little. I never tried it and I imagine it will be tricky not breaking or damaging the mirror.
The other thing I'm wondering is how are you seeing spots behind the mirror?
There should be a reflective coating on the mirror lens but you shouldn't be able to see spots behind the lens unless the reflective coating has been compromised and is de-bonding from the lens. You might have damage and need to replace the assembly.
Hope this helps...