If you are in a parked car and your back passenger door is partially opened and another car hits your backdoor who is at fault?

This situation is not one of those nice clear cut answers, but it depends on the situation. If your rear door was open enough to block traffic, or the flow of traffic, then you would be At Fault. By the same token if the person in the other car was driving in some manner that was reckless they could be at fault. This also depends on how your state laws work on impeding the flow of traffic.