Pull the glass off of the non-working mirror and make sure its the heated glass and you have the cables.
The last owner could have replaced it with non-heated glass or replaced the mirror with out the heating cables.
Go care full when pulling the glass off as it can crack or shatter.
I found it easier to tilt the mirrow all the down and my fingers behind the back and gently pull towards you. They can be a bit of a bit so a firm but gentle prising is needed.