Garmin seem to specify their GPS units operating in temperatures from 5ºF to 158ºF (-15ºC to 70ºC), which just happens to be the operating temperature of Standard TFT and Wide Temp STN

http://americas.kyocera.com/kicc/Lcd/notes/tempconsider.html

So the Garmin should work OK within this temperature range. At very cold temperatures the display will be very sluggish to change taking a couple of seconds for the LCD pixel to switch on and off.

There may be some GPS units that use Standard STN LCDs which may have a problem with the temperature extremes.

CFL Backlight issues would probably be more of a problem for the higher end models. The cheaper wide temperature STN LCDs are generally easier to read in daylight than standard colour TFT (which needs a white coloured backlight i.e. newer LED and CFL).

Of course the user using the GPS may not be operating at 158F.