The heater element is probably defective in the mirror. Sometimes the connector that the wires plug into the back of the mirror get resistive also. Move the mirror to the outside all the way and you can see the electric connectors behind the mirror. You can gently pull on the mirror and it will pull out. You can use a Digital Multi-Meter and ohm the mirror element to verify the element is open. Most likely it is. A new mirror is cheap from Volvo, just replace it, it's fairly easy.