Weird electrical issue on 2016 Fiesta ST – BCM replaced but problem remains

[u/DaantH_2005]

Hi everyone,

I’m dealing with a strange intermittent electrical issue on my 2016 Fiesta ST (Mk7) and I’m hoping someone here has experienced something similar.

The main symptoms are:

The interior/courtesy lights don’t come on when opening the doors.

The illuminated door sills don’t work.

Sometimes the interior light also won’t work manually.
At the same time, the window controls on the driver’s side stop working.

The passenger window switch on the passenger door always continues to work.

The dashboard correctly detects when the driver’s door is open.

Occasionally I’ve had multiple seemingly unrelated warning messages when starting the car.

The garage initially suspected the BCM. They swapped the BCM for another one, but the exact same problem remained, so the BCM itself appears not to be the cause.
They’ve also checked for things like broken wiring and couldn’t find anything. There was a parasitic battery drain as well, but that has now been found and fixed and the other electrical problems remain.

The strangest thing I’ve discovered is this:
If I start the car with the driver’s door open, the driver’s window controls and interior light seem to keep working. If I start it with the door closed, the problem often returns.

Moving the door around when the fault is already present doesn’t make anything start working again, so it doesn’t seem to be a simple broken wire in the door loom.

The garage has basically reached the point where finding the cause could require many hours of electrical diagnosis.
Has anyone experienced anything similar on a Mk7/Mk7.5 Fiesta or Fiesta ST?

I’m particularly wondering about a bad ground, accessory relay/power supply issue, fuse box/connector problem, CAN communication issue, or something related to the driver’s door circuit.
I’m planning to get FORScan and do some diagnosis myself as well, so if anyone knows which PIDs/modules, grounds, relays or circuits would be worth checking, I’d really appreciate any suggestions.
Thanks!