After some troubleshooting on half of quickclear windscreen on friend's 2004 Fiesta not working I have measured the resistance from the positive ribbon to the negative ribbon on the screen (with the wires disconnected). I have determined there is correct voltage reading on the wire that connects to screen ribbon and that the negative earth bolt makes good contact.

It reads 3.2 ohms.

Using  the formula  V=IR from my physics lessons at school (V=volts, I = current in amps, R = resistance in ohms) that means it only uses 3.5 - 4 amps which is far too low (and hence does not clear the screen).

I am not sure what current it should use but fuse is 30 amp, so lets say it should use 25amps. using V=IR that means the resistance should be about half an ohm.

Does anyone agree, or see some flaw in my logic? . 

The connectors on the end of the ribbon strips are sort of plastic block things. I do not know if the terminals are actually soldered inside those plastic blocks or crimped or whatever. Has anyone ever actually found that is exactly where the problem lies?  I am reluctant to butcher these connectors as not my car in case I have got it all wrong.

the other possible problem is where the ribbon strips fix to the screen but I can't do anything about that as you can't get to it with screen fitted..

Thoughts please

You may find that the heating wire resistance drops when it is heated - can you turn the screen on for a few minutes and then measure resistance on both sides to see if they tally?

you could measure the resistance of the element on the side of screen that is working to give you a baseline for comparison.

regardless of that , if you have the correct voltage at the loom plug , then the problem can only be at the ribbon that joins the glass ( corrosion or vibration ) or the element embedded in the windscreen ( high resistance break )

the first can possibly be fixed but a massive faff.

the second cant and needs new windscreen