All that ever seems to go wrong is the sensor, in the N/S door mirror.

Mick: I have the same problem, I did a mailing on 20 March, 5 April
and 8 April. I suspect Nigel is correct I now think it might be my
sensor. I have a Peugeot drawing which shows the leads from the sensor
going to the "intelligent slave unit" I would like to trace them
through but I cannot find this unit. I have measured the resistance of
the sensor at the wing mirror connector, it was about 500 ohms +- 50
ohms with cold/heat. Then I temporarily replaced the sensor with a
variable resistor and I found I needed 4500 ohm to 3000 ohm to get a
reading on the display of 55 to 70 deg F, so now I think the sensor
may be too low in resistance, best thing would be to change it.
I could let you have a copy of the drawing if it would help, I find it
a bit confusing!    Phone 01553671065      Dave Lake