Who is at fault in backing up side impact collision?

Whoever is backing up is at fault. No matter what the circumstances. Watchout!! Even if you are involved in a collision with an unlicensed driver who is driving recklessly, if you are backing up it will still be YOUR FAULT ! The question will answer itself if you didn't try to force the wording so much: Don't you think who is at fault "...when you back into the side of some one...." seems pretty clear?