Removing the 2 screws from the dash is hopeless as I tried it

it won't come out without a fight or a breakage so I changed mine with the shelf in situ, it's doable but very fiddly with minimal access, you'll have to undo the adjuster nut from the gearbox end to release that end of the cable, then up by the bulkhead behind the gearbox you'll see where the cable goes through to the inside of the car where there are 2 10mm nuts, be careful there as the studs can snap if you're too heavy handed, then on the inside of the car pull the carpet back as you have in the picture, undo the 4 10mm nuts holding the power steering ECU in place, remove the bracket then move the ECU out of the way for better access, then there will be a black metal cover with 3 screws, remove this then with a torch & on your back in a very awkward position thanks to Fiat

you'll be able to see the end of the cable where it hooks through the quadrant unless the hook has snapped in which case it'll already be free but you'll still have to remove the end of the hook still in the quadrant by removing the split pin, access on the Seicento is a lot tighter than the Cinquecento due to the dash & power steering wiring etc. I changed my cable when I replaced the clutch with the modified cable from Ricambio, the modified cable has a nut & bolt instead of the hook but I found a slight design flaw with the modified cable fixings, although the cable itself is a much better design as it has a round hole instead of the hook the nut & bolt setup looked like it was putting strain on the quadrant & access is even worse than with the hook so I did a little thinking over a cup of tea & what I did eventually I got a set of clevis pins with R clips from Amazon prime, pushed a clevis pin into the hole through the cable & quadrant from the back where the hook would usually go with a smear of grease around the pin then put an R clip through the small hole on the pin the other side, nearly a 1,000 miles in & it's still holding up, this also makes things a lot easier if I do have to change the cable again as all I'll have to do is pull the R clip out as I put it in a position where I can just pull it out with ease & push the pin through the quadrant with your hand the other side to catch it
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Also when reinstalling everything the black metal cover I mentioned doesn't need to go back in, I've left mine out to make it easier for next time so the jobs slightly less of a nuisance to do. Also it's good practice to regularly grease the entire mechanism to keep it lubricated which will stop the pedal from feeling heavy, I've done this ever since I've owned my Seicento & I've had no snapped cables...yet