I saw the trailer maybe 6-8 years ago. May have been a German language movie - the trailer was in English.
Post-apocalyptic. People hiding from very harsh sunlight. Kind of Mad Max.
Sounds like Hell (2011), a German movie directed by Tim Fehlbaum.
From Rotten Tomatoes:
As the sun scorches the Earth, threatening to wipe out all life on the planet, desperate survivors Philip, Marie, and her younger sister Leonie race to reach a water reservoir rumored to be nestled deep in the mountains. Making the acquaintance of a lone mechanic named Tom as they head towards the mountains in a specially outfitted car, the trio must fight for their lives after being drawn into an ambush.