Unplug the connectors of the window switches and try what the fuses do. If they don't blow, you can plug connector after connector to see when the fuse blows. It will point you in the proper direction to seek.
The fader switch has a LED for the dash illumination; maybe they messed up something with that (although I cannot imagine what).
The 1991 300D power windows are quiet easy to work on.
Maybe one of the connectors in the doors is being shorted (I believe that the US models have speakers in the front doors???). The 2 wires of the electric window motors are attached to the wire harness (to the switches) by a screwed connection in the steel door panel.
with a minor 600+k