Pretty simple relationship really. The key elements are the door and the ignition off. The diode allows power to the seat when the door is open without the key on. It separates the normal key on operation. Without the diode you won't be able to move the seat with the door open and the key off.

Not sure how the diode issue occured with the relay. Leave it out till you verify that is the only issue and then replace it. It is probably shorted (conductive in both directions)

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician