passanger side mirror replace glass

[brad`]

ok so another question haha

the passanger side mirror of my brothers car is no existent for god knows how long haha,

is there any way to replace just the glass or do u have to replace the entire mirror unit.

i was thinking of just getting a new peice of glass cut and just silicon/glue it in place of the old one just not sure of how reliable it would be haha

cheers
brad`