The exterior air temp sensor is mounted in the o/s mirror it's a little nipple that pokes out the bottom of the mirror pretty sure it's a ntc (negative temperature coefficient) sensor a bit like the missus the hotter she gets the less resistance she has ! , mine has a spaz from time to time sometimes it can be 10 or so deg and it says it's freezing , most marques you can get them seperatly on eBay but a quick test with a multimeter through varing tempretures should confirm it