My 87 190D works the same way. The power seats only adjust with a front door open. The Electrical Troubleshooting Manual shows on page 138/10 the diode. The diode isn't listed in the component location list and I'll be darned if I can find it. The diagram would lead you to believe it plugs into the fuse panel (Electrical Center) but all the devices on the panel are accounted for. Does anyone know where it is on a 201?
1984 300D Euro