EXOTHERMIC WINDSHIELD DEFROSTER?

I've been thinking about rigging a device to automatically defrost my car windscreen about half an hour or so before I get in the car. Since it's got to be able to turn itself on while I'm still asleep, it's got to work off batteries. I think that pretty much rules out my first idea of a heating element / fan setup.

My next idea was to use some kind of exothermic reaction and either directly transfer that heat to the glass by conduction, or with a similar convection / fan setup as previous.

My questions are:
1) Does this sound at all feasible? If not, any better ideas to achieve the same aim?
2) What reactants would be good to use? The spec is: It has to produce a decent amount of heat over a decent enough period to give (at least a decent chunk of) the windscreen a chance to warm up and defrost without cracking the glass in the process. Also the products of the reaction should preferably be non toxic and easy to deal with (no masses of liquid, poison gas etc!)
3) Has anyone heard of anything like this being attempted before that I could sneakily steal?Ta very much.Adam?