That happened to me during the first week of owning my 172! Some fat woman in a sliver 02 plate astra, my write of way but she jus kept coming and BOSH, smacked my wing mirror an the glass cracked. Tried to spin the car round to go get her but she was gone!
Anywho, glass just pulls out its on plastic clips. Was £20 for a new glass from renault and mine was heated.