How sensitive is the Messenger to heat sitting in the sun on the dashboard of my vehicle?

When my wife is driving to pick me up at the trailhead and is out side of cell phone, I would like to be able to have her set the Garmin unit on the dashboard of the car so she can be notified of any messages I might send to her while she is traveling to pick me up. This area gets pretty hot. I do not want to damage the unit. any ideas or suggestions?