There are a lot of sources for indoor/outdoor thermometers, frequently including clocks.
Radio Shack has one that simultaneously displays indoor and outdoor temperatures. IIRC it normally sells for ~$20, but has been on sale for ~$15. It's wide, probably about 3", but pretty short. It'll "fit" beneath the pocket below the radio (non-NAV) if you don't have something else there (that's where I stick my CD-changer's wired remote).
I just picked up another model at Target that's narrower and higher. It's gray, not black like most designed for use in cars, but still not too unattractive. I think it was about ~$10. Look in the housewares section if you don't find what you want in automotive.
Many hardware and home stores also have indoor/outdoor thermometers that will work, especially if you're willing to mount on the left side of the steering wheel.
I suggest that you mount the external sensor somewhere in the grill area, not behind the bumper. You want the sensor exposed to ambient air and not subject to heat radiated from the engine. (I had one mounted too close to the radiator on another vehicle. In sub-freezing weather it'd tell me it was over 100 when I got stuck in traffic.) Also, mount it so it isn't subject to sunlight, e.g., the lower slots, perhaps on the bottom of a horizontal slat.
Mel