I hope this helps someone. On my 2006 V50, it's the passenger side rear mirror where the sensor is located.

Plastic pry tools, a 13mm socket, T25 torx, a thick paper clip and less than an hour.

Simple "How To" video showing how to easily replace the wing mirror mounted Ambient Temperature Sensor in a Volvo V70 - and with very few tools. Anyone can do this!! Is the same procedure for all of the vehicles on the P2 Platform (S60, S80, V70, XC70, XC90)

I've had a go at this today on my V50. There's a speaker in the internal cover that fits over the internal access to the mirror. The nut that holds the mirror (presumably) looks accessible behind this fitting but it doesn't look to me like it's the way to go. Further investigation to follow. edit: it appears to be part of the door panel and comes off with it.