Ok, on the later 96MY 300 series vehicles a wiring loom fits all… as you have found out
Purple-Blue is shown in some wiring diagrams for the drivers door only which still goes to the MFU
Rear fog lights and interior lights use the MFU for their switching, so that would suggest the MFU could have an issue… assuming the replacement wiring loom is correct.
The MFU can be put in a diagnostic mode which tell u what circuits are good or not only… work or don't work, but there’s always a chance that the MFU wont access its access mode to do its test due to the issues.
Without me checking but I expect that the heavy brown wire is a 12v supply to the board from the engine bay fuse box.
I need look what the dash book symbol is for, read the owners handbook, maybe.
Also the fuse numbers that have blown are?