I am not at the diagrams so I will have to wing it. First if you have found the drawing for the power seats and the diode, this is what you need to find. The first pages of the ETM (electrical troubleshooting manual) would show the layout of the fuse box.
The fuses are under a cover that easily comes off. There is a further cover which comes off with a number of screws. this exposes a number of relays. Find this illustration in your CD. The diode is the only device of its kind in the section that has multiple relays all plug in with multiple pins except the diode. It is at the exact left rear corner and has only two pins. Pull it out and see if the symptom (radio plays with key off) still exists. If it doesn't make any difference then we need to think up another hypothisis, probably based on a little further checking.
As to tracing the circuit. Go to the drawing that has the diode V1 (power seat drawing of source power - more than one page of power seats). You will notce that fuse #9 is sourced at term 30. That means it is hot all the time. Connected through the door switch and then the diode wiring is fuse #2 which is only powered by circuit 15 (key on). If the diode is shorted these two circuits become powered whenever the door is opened. Now find the drawing, also at the front of the book, which describes the power flow. It will show fuses and which cicuits they power. It will also show where they get their power and which are connected at the source power side. Fuse #2 is bussed to fuse #1 on the source side and fuse #1 is the switched power fuse for the radio.
But before any of that go out to the car and pull that darn diode and see if it kills the problem. I am dying of curiousity.
Bosch Master, ASE Master, L1
33 years MB technician