electronics do this when exposed to direct sunlight for hours at a time. not sure why it's that hard to figure out. if you've ever had a gps or radar detector, you know not to leave them up when you're away from the car, and that they get extremely hot when exposed on the windshield or dash.
it's not your charger, it's not your dock... it's the climate. if you're on a 4 hour trip you don't need it on your dash for so long, anyway.
Thank you oh wise one.... Its not that its hard to figure out that electronics exposed to direct sunlight get hot, lol. Its that the Droid automatically shuts down charging at certain temps. I/we were not aware of this, thus scratching our head when our phones would not charge on the car dock.