My heated front screen has only ever had about two of the heater elements that worked but yesterday i noticed that the button for the front screen does not light up any more .So if the last two heater elements have given up the ghost would this stop the button from lighting up as if the circuit is broken ? 

The element breaking could've blown the fuse, hence you don't get the light on.

Thanks for the help ,the fuse is fine i swapped it with the rear window demister fuse   ,so even if all the elements  had broken down i take it the light would still come on ? If the ribbons that supply power to the window had broken would that stop the light from coming on ? if not i have another electrical prob🤬

Do you know how to measure the resistance of the window? If you can prove continuity then your fault lies elsewhere and is quite likely to be the relay. If the on light takes its feed from the windscreen side of the relay which would seem reasonable, that would be a pointer to the relay not working.

Is your battery alright?  Heated windscreen takes a huge amount of power so is restricted when the battery charge is low.

That said, I'm not sure I'd bother with it at all if only 2 elements are working!  

I've always found the the heated windscreens will only work if the engine is running.

43 minutes ago, Bobr said:

I've always found the the heated windscreens will only work if the engine is running.

Yes, the handbook makes the point that the engine should be on before using the front or rear heated screens, also that they may not operate if battery charge is low.

Have decide on a Nextbase 322GW dashcam with the rear view attachment.   My insurance company are offering me a 25% discount if I use this model.

Thank you for all the advice ,no i do not know how to measure the resistance of the window and yes the battery is fine nearly brand new , where is the  relay is for the front screen? when i press the button for the rear screen i can hear a click from somewhere around the drivers footwell ? one last thing are there any fuses i should check in the little black box next to the battery ? 

Find an online manual and find the relay locations from that. It will also give fuse information. If measuring resistance is something you haven't done before, you will need a meter which is anything from £10 to £1000+ (Spend around £20 and it will do for life. The ones I buy are for test lab use, calibrated to international standards and very expensive. They do exactly the same job and no better.) Read about how to use it online. It is not difficult, even if you are not familiar with it.

The relay is in the fusebox behind the glovebox assuming the Fusion is the same as the Mk6 Fiesta.  R2 I think.

The back of the removed glovebox will have a decal showing fuse and relay positions - two 30A fuses (42 & 43, Green), and note that rear window is 20A(26,Yellow), and dead right Tom Golf, R2.  We have a Fusion and a '07 Fiesta, and I'm sure they are the same.  The LED "ON" indication is actually controlled from the General Electronic Module (GEM), and along with the relay coil, gets its supply from F38 - as do other things controlled by the GEM.

(Tom Shepp - No doubt your meter is True RMS ?)

Yes Paul, amongst other things such as intrinsically safe and capable of measuring four terminal resistance down to bits of a milliOhm. (gets swapped out every five years ' cos the microvolts range drifts on 'em.)

Thank you all for your help and pointers I have got myself a handbook and a voltmeter  today is the only chance i have had to look at it so we will see how i go