As a closed, but not sealed, unit the rear view mirror had collected some dirt over the last 30odd years so I took it off the car to clean.
The mirror unit isn’t too hard to open, the plastic surround clips into the metal shell with three tabs – one on the top and two on the bottom.
Once opened the metal shell did have a bit of dust inside which was easily cleaned out.
After removing the sellotape that had lost most of its stick, the mirror and glass also aren’t too hard to remove from the surround but care is needed not to damage them.
After a thorough clean, assembly is the reverse of removal. Of particular note is the prismatic anti-glare mounting. The mirror is at a different angle to the glass and held in place with a wedge shaped tongue running round the inside of the surround.
The mirror and shell fit back together relatively easily with only minor coaxing required to get everything back in the right place. Note that the surround is fairly soft plastic so care should be taken when refitting so as not to damage it.