The aux warning system/graphic display has two ECU'S, there is one behind each "kick trim panel", the drivers side one is the "Bulb outage monitor".

Like I say this ISN'T a ECU problem, the problem lies with that circuit, so the first thing you want to be checking is the switch on the affected door!

This it NOT the 'Prong' switch by the hinges, that operates the interior lights, it is integral to the door latch and you can't see it without removing the door card.

Your problem is the horns themselves or the wiring I should think, the fact the relay clicks when the switch is actuates shows it to be working!

As said for the horn location, test each horn by wrapping a rag round it etc and applying 12V, if both work then you have a wiring problem.

Yes if you have condensation on the PCB this could cause the fuel gauge issue, in any case the gauge should always stay at the level when the ignition is switched off, and update when you first switch on after refuelling.

The cluster isn't that hard to remove if you wanted to inspect.

Martin