Looks like there is a few spots on the roof ,so maybe you could try a few things up there to test ,if that is rust stain maybe vinegar mixed with some water ,or CLR that takes rust stains off .I am sure someone has a product that will work. You could also try some polish to se if you can work it out but then you would have to do the whole truck as it will be shinny . My truck has been on sabbatical in the garage for a few years and was very dusty and marked up from working on it so I used cheep polish to go over it to get it clean after I wiped it down
