W210 Outside air temperature sender

[AlSchultz]

The outside air temp gauge on my 99 E300 has gone haywire. With outside temperature in the mid to high 90s my display on the dashboard reads something balmy (or barmy?) in the mid 70s. Where is the sender for that feature? I'm told it's somewhere behind the front bumper but that covers a lot of real estate. How can I find and is there a way to test it to see if it is the problem? Thanks!