Power is supplied to the vertical line each side, the horizontal wires are thus all "in parallel" and not in series.

Just the same as your household light bulbs in a multi-bulb fitting, if one blows the others stay on.

You can buy reapir stuff and fix it if you have a steady hand or use masking tappe.

Halfords kit cheaper or conductive paint off e bay. Use masking tape like madf says. It worked for me where a dog had scratched through it.

I bought a new car once and,tho' the protective coating on the elements was unbroken you could make out breaks in the conductive coating under it-the rear window was replaced under warranty but the dealer queried why I had waited so long before finding the problem.I bought it in the summer!!!

Doh! I can't see any breaks so the breaks could be underneath protective coating. I just don't know, as I said I haver these spots mid way along the wires where it is working. So sod It I've bought a bottle of anti mist stuff , see how it goes.

It would seem you've got partial breaks in the elements. They're not completely broken, but damaged enough to limit the current. The 'hot spots' are the damaged points. The damaged point provides a high resistance path and is where the element gets hottest. There won't be enough current flowing through the element to make it as effective.

Clever! If that's the case all I have to do is mark those hot spots on the glass when the window is still misted and then I know where to apply the repair paint. Now we are getting somewhere.