these cracks have occured because of the dark colour of your door and where its possitioned,my guess is that when its sunny your door is in direct sunlight for most of the day,with it being dark plastic it attracts the heat causing your panel to swell and in some cases crack the panel,which has happened to your door so if you get some dark silicone and fill gaps this should work as it will stretch when door swells again,if door is guarenteed this should be covered.hope this helps
shaun sdwupvcrepair.co.uk
May 2008