Got the dreaded "Soot particle filter full" message again yesterday. Had a good look at the rear screen heating controls and there is no indication on the dash that it is on at all. Perhaps engine management is able to switch it on without lighting the LED in the button. I'm going to connect directly to the heating element. For the benefit of anyone else wanting to try this, remember to use a 1K resistor in series with the LED, otherwise you'll fry the LED. Also, the LED is a "polarised" device. If you get it the wrong way round it won't light, so test your circuit on the car battery first to make sure you've got the polarity right. The advantage of using a LED is it will draw very little current and won't generate any heat. You can also use a small conventional bulb if you don't have a LED or resistor. By small, i mean something the size of a couple of match heads, the kind they use to light up displays in car radios etc. This would get hot but not hot enough to burn you or whatever it's lying on.
A last word of caution. The 12v supply to your rear screen heater will be able to deliver a fair bit of current. Take great care not to let the wires you attach to it short together.
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