The tray under the wipers is held down by what appear to be plastic screws but they aren't. Put something wide but thin (e.g. a sharp chisel or a thin flat screwdriver) between the screw head and the collar underneath it. Prise the two apart. The screw part will lift about 5mm then won't go any further. Now you can pull out the whole thing, including the collar. To replace, put it back in the hole and push the centre part in (just with your finger).
Edited to add: Wire cutters may be useful to pull it up. Just squeeze gently, enough to grip it, not enough to cut it off. If any get broken, replace with standard trim clips from Euro Car Parts, don't bother with Fiat as they want over £1 each for them!
The washer fluid arrives in a thin (5mm-ish) black rubber tube on the passenger side, near the pollen filter. Then it goes into a black plastic T-piece, with the two branches going to the left and right jets. You will have a blockage between this T-piece and the jet itself. There's probably a bit of something stuck in the back of the jet. You might need to suck it out somehow, or blow something into the jet while the pipe's removed - at either end, just so there's somewhere for the crud to fall out.