The system is quite a bit more complicated than just a timer module.
When you push the defrost button, the control head sends a class 2 data signal to the rear integration module (RIM). The RIM then engages the rear defrost circuit. Once this is done, the RIM sends another class 2 data signal back to the control head instructing it to illuminate the "On" indicator. I would have to do further research to determine whether the timer is part of the control head module or if it'* managed by the RIM.
Further to this, there is also a class 2 data signal sent to the driver door module (DDM). This is a gateway module for controlling all door functions. The DDM applies power to the left outside mirror element while at the same time sending a message on a simple bus (connecting all four door modules) to the passenger door module instructing it to turn on the passenger outside mirror element.
I'm assuming that you believe the rear window shattered because the rear defrost remained on. While I suppose this may be possible, I'm not really sure how likely this is. There are circumstances when it remains on constantly by design. If your speed remains above a preset level, it will remain on to ensure your mirrors remain clear. This really only applies if you have heated mirrors, but I don't believe the system actually differentiates whether you do or don't. I believe the reason these turn off is not due to "excessive heat", but because this system generates a heavy load on the electrical system when it'* active. At speed, your generator has increase output and it'* not as big a deal. At lower speeds it can antagonize the electrical system a little more.
StingRay Bike w/cards in the spokes, Weeeeeee!
Last edited by ddalder; 01-25-2009 at 12:42 PM.