I used to have an Austin Metro. British designed and built so most parts fell off at one point or another. However the rear view mirror was easiest to solve - Epoxy resin. Two tubes of chemicals come with it - apply one to the mirror side, one to the glass the other. When they make contact a chemical reaction causes the two surfaces to stick. Trust me - it will outlast the rest of the car! You can buy epoxy resin in Halfords.