That wiring looks a little bit unusual. I wonder if the dash had problems before and someone tried to diagnose the fault?
There is probably some way to test the plug.. but I have no idea how, nor how you can find out... but you never know, someone on the forum might have access to the Fiat diagnostics instructions, if those go into that level of detail.
The clocks usually fail because one or more of the tracks on the PCB can corrode and if they're not perfectly formed, or thin or just brittle after a decade or more, that can impede the signal. They need a clean up or occasionally a new flash of solder to restore the connection.
The "graphite pencil" fix is just a way to bodge a repair if you can see a break in one of the circuit/tracks. You basically use the pencil to re-create the connection by laying down a line of graphite between the breaks. It may work and it may not.. or last just 5 minutes. Probably only one for Apollo 13...
If it was me, I'd get hold of some second-hand clocks and without fitting everything back together, just stick them in there to see if they work. Clocks need be be proxy-aligned to the Body Control Module, so "foreign" clocks will flash the odometer (and that will show the mileage of the donor vehicle) but otherwise they will be a straight swap and just work, if your wiring is okay. There's always a market for Stilo clocks so you can sell the spare set afterwards and get most of your money back.
You don't particularly even need "diesel" clocks... "petrol" clocks would work too, although the numbers printed on the rev counter will no longer be in the right place. The rev-counter needle will point to where the diesel revcounter printing would be.
Anyway, if the clocks check out then it's the wiring... and if the "new" clocks give the same errors (assuming you know for sure that they are not also knackered
) then you'll know it's the clocks.
Otherwise/in any case, you could do worse than find
yellowstilo, a member on this forum. He'll be able to test your clocks to see whether they're any good. If you ask him nicely he might not even charge you if the clocks are fine... but don't tell him I said that...
Only tiny drawback is that he's in Holland, so it's a bit dearer to post the clocks to him and you probably need to do it sooner rather than later in case your clocks get impounded by Customs after Brexit day... (although technically they would be a temporary export, so not subject to import duty).
Make sure you send the clocks tracked.. Royal Mail (or Dutch post office) lost my original clocks... My second set (which also failed shortly after I fitted them) were repaired by Yello about 3 years ago now and are still (touches wood) working fine.
Ralf S.