I asked Tomtom support about leaving the iPhone car kit in the car at night in the winter (I live in Ohio). Here is the answer I got.
The operating and storage temperatures for the TomTom devices are as follows:
-4ºF to +140ºF / -20ºC to +60ºC
So it can withstand the extreme temperatures inside the car. The only recommendation we would like to make is to keep the LCD screen of the device away from the direct sunlight, as it might damage the LCD screen.
Sounds like they're talking about one of their GPS devices, but its probably the same.