@joko, My understanding is that when a person registers as an organ donor, it is honored as a legal agreement upon his/her death. He/she has already made the decision while still alive, and thus it will be valid upon death. However, the family/loved ones, are always informed of the persons decision before the donor process is carried out.
My friend wants to donate her body to science, and I remember her telling me that she is already registered with a particular medical facility to do that and her family is aware and in agreement. She said that has to be pre-arranged.
An interesting thing happened just 2 weeks ago when I was at lunch with this particular friend and a few others. We were discussing the new driver licenses and sharing our awful photos. I noticed that she did not have the Organ Donor notation on hers. That bothered my, until I remembered our earlier conversation.