A quick anecdote.
I have had a Fujitsu Stylistic 3400S tablet mounted in my car for the past 3 years, two of that in southern California, and the past year in south Florida. Windows are non-tinted.
The machine sits on a post in front of the dashboard at present. Over the years, the machine has been rained on (it's a convertible), it has moisture condense upon it every time I leave the top down at night in the summer, it has direct sunlight shining on it all day when I'm parked at work with the top down, it suffers through immense heat when parked outside at the airport for weeks on end (sunlight on a black convertible top = really quite hot), and is subjected to pretty serious shock and vibration, as it's rigidly mounted to the floor and my car's suspension is quite stiff.
For the most part, everything still works perfectly.
The battery gives about 5 minutes of runtime from a full charge, but then, it's the original. 5 minutes is plenty of time to hibernate, so it's not a problem. When it is very cold (say, 40°F or below) the display is dim, but has never completely failed to energize. The touchscreen is somewhat unresponsive, however this is sort of a known issue with these. When I hit a particularly hard bump, the display will occasionally go on the fritz, requiring a reset. This happens once every month or two, on average. The hard drive, surprisingly, is still error free and performing perfectly. Likewise, the external connectors (power, USB, PCMCIA) have suffered no corrosion and still work perfectly.
In other words, it seems no worse for wear for a computer that is nearly ten years old.
The only downside is that the supposedly outdoor viewable screen isn't all that good. When I have the top up on the car, then it's great. With the top down and sunlight hitting the screen, it's difficult to read. Impossible if I have sunglasses on. For a hardtop car however, it would be fine.