The heated rear windscreen is not working on my 7th Generation 04 diesel tourer, 1st winter in my ownership. The button led illuminates , all the relevant fuses, relays are intact.There is continuity across all the heater elements in the glass, no visible damage to them, and continuity between the connector tabs for each electrical connector in the glass , drivers side to passenger side.However there is no power to it at all from any of the connectors key in position switch activated, so am now looking for a known likely location where any of the wires feeding both sides may fray, or be stressed / damaged. Suspect tailgate area . I can see the heater element is in circuit with the mirrors , and climate control , indicator stalk they all work perfectly mirrors heat up on demand.It is the only electrical device that does not function. The radio (that I don't often use, prefer cds ) is also now subject to a high pitched whine that alters and matches a change in revs (alternator output ) , but only when the headlights are on, otherwise interference free.Not sure if this is recent to tbh. I read somewhere that the radio amp in one of the rear trim pillars, may or not affect the rear heater element if faulty, but cannot see the connection on my wiring diagrams, if this is the case. Anywhere / anything else I should be looking at ?. Not really that urgent, but it niggles me to death that is does not work after the grief of my recent DIY timing chain change adventure, in the first real frost since I bought it.