Join Date: Jun 2006
Location: Eastern ON Canada
Car: 07 OBW 2.5i D-4AT
Feedback Score: 6 reviews
The rear defogger and mirror heaters are electrically dependent; the mirror heaters are powered, in part, from the rear defogger system. But the wiper deicer (actually a heater in the lower part of the windshield) is not dependent on the defogger system -- it can be operated independently. So the odds of both systems failing at the same time would seem to be low.
There is a common factor to all three, namely the "timer" that you mention. (This is part of the Body Integrated Unit, or "BIU".) However, if either the wiper deicer switch or the rear defogger switch is turned On, and the "on" light goes out after 15 minutes or so, then the related timer in the BIU is working, and that would seem to eliminate the BIU as a factor. (Also, the fact that the relay(s) seem to be working (click) when either the wiper deicer switch or the rear defogger switch is turned On and Off manually would tends to verify the continuity of the timer circuit.)
The broken wires going to the rear hatch can seem to have odd effects, some totally unrelated. So it's not outside the realm of possibility that a broken wire is affecting all the heaters. But I'm less inclined to focus on this at this point.
Which brings me to a question: How are you checking if the heaters are working? I ask because in extreme cold, the electrical system is under more load (battery capability is reduced, alternator has to work harder) and therefore the heaters will not be as effective as they might be at other times. It can take a lot longer before the effects on interior surface mist, and exterior snow or ice, tend to show. All the more so if the battery or alternator is weak, or there are poor connections in the electrical system, to begin with.