have you purchased a new one?
The ambient temp sensor is behind the licence place attached with one 8 or 10 mm retaining bolt. From there it snakes along the driver's fender to just under the fuse box (4 or 6 screws holding the fuse holders and assembly down). There is a connector under there that you'll have to disconnect and reconnect the new one.
On a side note, you have other options. The ambient temperature sensor is simply a 3300 ohm thermistor (resistor designed to maximize resistance changes with temperature) You can order these from
www.mouser.com or
www.digikey.com for less than $1 ea. Then you can simply snip the end of the OE temp sensor and attach the new thermistor. I drilled the reminants of the old thermistor out of the metal enclosure, soldered the new one onto the two wires, dumped some epoxy in the freshly cleaned out enclosure and stuck the new thermistor in that. Then bolted it back up under the license plate. Good as new.
Also, if you want to test to see if the temp sensor is really the fault, remove the fuse box (take the cover off, then take the ~6 screws out and lift it up) find the ambient temp sensor connector (following the wires from the sensor) and unplug the sensor. Use a 3.3k or 3k ohm resistor across the two pin connector and the gauge should read ~70F or so.
Good luck!