"Right there was when I knew that my dad was good."
Now, let's apply what we know from physics to unweave this story. Why would you be able to see a rainbow in your rear view mirror but not looking directly at the bed of your truck?
Well, because rainbows consist of numerous refraction and reflections of the sun among many various drops of water, within the drop themselves. And there's a bow that appears at your feet when you look down (usually with a hose you can see them down there) but they only occur when you look down at a specific angle (42°). And this is what changes dramatically between looking behind you and looking in the rear view mirror, the angle at which the light is is being reflected from the sun. Shift to less steep angle and the rainbow might not be there (and the angle seems spot on considering the angle from the truck bed to rear view mirror).. Because rainbows live in light, not in space. This is no stranger than you being able to see the sun by looking behind you but not seeing it in your rear view mirror.
Just as you can see what looks like like a wet road on a hot day quickly drying, some hundred feet ahead of you, while you're driving. But if you look straight down, you'll never see it look remotely shiny (the heat causes the light to refract and if it goes through enough atmosphere you'll see the sky where the road should be).
See, his father is just actually dead, and a rainbow doesn't change that. While the story isn't the sort of cheer you up magic one would claim, the rainbow is just as pretty as it ever was.. Science isn't cold. Rainbows are pretty and dead people are really dead. Science is about reality.