Skoda fabia dashboard lights fault

Hopefully this will save someone some money. All the dashboard lights (instrument cluster, radio buttons, heating control, etc) developed some weird intermittent fault, where sometimes their backlights would not work at all, preventing me from seeing important things like the speedometer, when driving in the dark. There was some strange interaction with the windscreen wipers and possibly the head lights. The Skoda garage wanted £80 or so just do go through their diagnosis process, so I decided to try to fix it myself. The first thing I found was that the dash light dimming was not working at all. This lead me to suspect that the dimmer was at fault.

Get this switch panel out by pushing the headlight switch IN from the 'off' position, then turning clockwise slightly, before pulling the whole assembly out (I found this out in one of the many fact-filled threads at www.briskoda.net).
Getting the dimmer control PCB out was quite difficult:

The dimmer control came out by prizing the plastic where the dimmer 'module' goes into the front panel with a small flat screwdriver. I couldn't help but damage the plastic, but it's not visible in normal use. I had to put 2 screwdrivers in at once on one side, releasing that, then go on to the opposite side.

The dimmer module itself was also a pain to get apart. A similar technique to above, again using two very small screwdrivers, allowed the two halves of this to come apart (the top half comes off vertically in the image).

The module came in half, and looked like this. The PCB is held in place by small plastic hooks that can be bent out of the way slightly. Turn the board over to reveal...

An Infineon BTS730 high-side power switch with built-in PWM generator. I was able to probe this with a multimeter, and found that in the fault state, although the device was receiving ~12 volts across its power rails, it did not seem to be doing anything (I also made sure the control potentiometer wiper was varying as expected, to rule out a dirty potentiometer track). I replaced the part with another one from an (incompatible, see below) dimmer module I bought on eBay, and it had the same problem. I decided to simply bridge pins on the chip so that the supply voltage is always passed to the LEDs in the dash board. This meant connecting pins 7 and 8, but I decided to bridge all of pins 4-17:

Putting it all back together, the dashboard came back to life. The dimmer of course doesn't work, but ~6 months later everything is still working well. I never had the dimmer below maximum anyway.

I did try just getting a spare from eBay and the scrap yard. A Fabia light switch panel from eBay had slightly different version, which turned out to have a different connector (I stupidly bought this before taking my own one apart):

As well as different internals (although using the same IC for dimming):

I tried taking the IC from this one, but the problem remained, so presumably the Fabia I took it from had the same issue. I also got what looked to be the same thing from an Audi A4 Estate in the local scrap yard (a lot of parts like this have Skoda, Audi, SEAT, and VW stamps in the plastic moulding, as many parts are shared between these companies), but this turned out to have completely different circuitry, despite having a very similar connector:

Like I said, hopefully this fix saves someone some money at some point. If you're going to alter your car's electrics, make sure you have some idea of what you are doing. A short circuit could easily cause damage to some other system in the car. I have not described all the measurements I took to find this fault; you will need to make sure you are happy with what you are doing if you are going to try to make this repair yourself.