Stands to reason that you have two way connection through the power seat diode V1. This is a two pole device located in the fuse box. It will connect fuse #9 which is hot all the time to fuse 2 which is connected to fuse 1 and that should only be hot with the key on.
Its purpose is to separate the ability to work the seats with the door open from common circuits (when acting properly as a one way device [Diode]).
Bosch Master, ASE Master, L1
33 years MB technician