Breezy_Pete

My next suggestion is to clean the contacts of the adaptor connector that goes onto that board-mounted connector. In this photo of yours there's some crusty green corrosion residue in a few places, and I think I can see some on the contacts of the fork that goes onto the driver's window switch contact pin on the circuitboard. Ringed below. Also it is probably worth gently and carefully squeezing these contact pairs together, to improve the contact pressure when it is fitted back down onto the circuitboard. The legs of the forks are quite long, so I think they can open up a bit over the years which maybe unhelpful. You may as well clean up all the contact surfaces of all of those fork pins while you're at it.

VIN info yielded: No start/stop implementation from factory. Built with a standard lead/acid wet cell battery (61AH/330A), conventional alternator with fixed voltage output. From my understanding that means there is nothing that could be coded relating to the battery.

Meassage me with your VIN and I can check what battery and charging system type it was built with, if you wish.

Only start/stop cars as far as I know, gearbox irrelevant.

I'll message you about getting a known good motor to you as a diagnostic aid.

I think mass means earth.

Needs a bit more cleaning on topside to see clearly, but possible short to ground plane here, which is the connection that is giving trouble, I think:

Skoda parts list two superseding part numbers, with latest being suffix-T by the looks of it: https://www.skoda-parts.com/spare-part/5ta839015t-door-lock-rear-left-skoda-44668.html

The pins I mean :

Most of the pins are easy to solder-suck clear of solder for removal, the hole size is quite generous for the pins. The two exceptions are the ground pins, which take a lot more persistence. As we view that photo, they are the two nearest the camera in the right hand column. It's well worth making sure every pin is free before levering the connector body away, otherwise any pins that still have webs of solder may get pulled through the plastic body of the connector.

Does the drivetrain CAN bus start working again if ABS module is unplugged?

The board damage - if present - won't be visible until the yellow-ringed connector is removed.

Some versions seem to have rear door locks wired direct back to BCM, others via door modules. If rear windows are electric and are working, it's probably not the LIN wire.

I can probably find wiring info for you, but will need VIN. I'll message you privately for you to give me that. On a lot of models, the rear door modules are LIN sub-modules of the front door modules on same side. This has a single-wire data connection between front and rear. This wire may have broken at the point where the driver's door loom flexes, rather than where rear door loom does. Have you insected for wiring damage at either of these points?

I'm thinking more about the alternator than the battery.

Monitor battery voltage in situations where the problem is occurring. The extra power being used by the radiator fan (plus cabin fan) and the extra heat from A/C condenser may be causing the charging system to struggle. Something with voltage display plugged into the cigarette lighter is maybe the easiest way of looking at this while driving.

Manual. Yes, it is spring assisted I think. The front two magnets were loose and free to spin out of the correct orientation, for reasons unknown. Rear two seem much more firmly retained and take positive effort to rotate. Gaffer-taped the front two to each other and the top surface of the blind for now. Doubt that's going to stay there for long, but I did at least alcohol-clean the surfaces first.

Just for extra clarity, the solid-state power controller for the fan is the part ringed in this photo, from an eBay listing of the part number quoted by the OP. Ebay listing: https://www.ebay.co.uk/itm/406703562083

New development, thanks to the main driver having just used the car and remembered something. 😁 Sunblind that covers the panaromic roof has been playing up in the heat, sometimes the front half opens while driving or when a door is shut, with a fairly alarming (geddit?) bang when it hits the end stop. She just popped out and found front half of the sunblind open, and was fairly sure it was shut last time she parked it up. Maybe the heat is making it sometimes open while the car is unoccupied and locked? That would be a very reasonable cause of alarm actuation, I think. Off to investigate the magnets that supposedly hold the blind shut...

The car's been in regular use, mixed journeys, for well over a year now. Rarely sits idle for more than 24 hours. I'll let you and everyone else know if there turns out to be anything wrong with this battery, or any further issues with the alarm. Just looked for any evidence of bugs near/in the interior motion sensor; nothing found. Car alarms are such a waste of time; so happy that my Fabia was built without one.

I don't have a battery tester, I will just be patient and see what happens. Could be something daft like a spider living near the ultrasonic sensor.

Brand new battery fitted two Fridays ago, the day of the alternator failure breakdown.

New alarm horn fitted. One false step in trying (and failing) to remove a shear-screw that looked like it needed to come out. [I cut the slot, wasn't sufficient]. Undid the M8 nut in the vertical face of the bracket, and found that there was then enough room to wriggle it out without anything else being undone. I did remove the rainwater shroud from above/around the cabin air intake to make more room. Allowed for better cleanup of scuttle too. No sign of anything wrong in old unit though. NiMH battery at 6.8V, which isn't horrible for a 5-cell arrangement. Capacitors on board all measured OK in value and tan-delta. Nothing else obvious. Will just have to wait and see.

I think all climatronic fans are going to have solid-state power control. If this wasn't the case, the electronics wouldn't be able to vary the fan speed independent of user input.

I'll cut the old horn unit open after the swap, to see if there's anything to see.