This is only a guess but perhaps the electrical connection to a moveable piece of glass is too precarious

Bad guess. Electric defrosters are accepted on the rear window since the wires do not appear visible, being so far from the driver. Wires on the side windows would be more apparent, and more distracting, decreasing visibility in normal circumstances. Legal liabilities abound.

Hot air is your option of choice.

Sand, my comment was meant to be flippant, not sarcasm. No offense intended.

No offence taken. I've heard worse.

Wired ribbon = simple electrical connection. Yet complex. Miniaturizing the wires or perhaps finding some way to use clear materials (fiber optic/thermal?) would help here. Even with wires in the way, your sight plane would be far beyond the window, so the wires wouldn't obstruct much. Think screen doors.

clear heaters are a known technology. I think the reason not to put more electric-heaters in any car has to do with complexity and amperage draw.

If you just start the engine, then engage the blower motor, the headlights, the wipers and 50 amps of heaters, it might even slow your idle speed enough to kill the engine.

Also, doors open, window move--wiring to moving objects results in complexity with many avenues for failure. If you're fixing cars on warrenty, it's easier to not design problems like this.

transparent heater:http://www.minco.com/products/heaters.aspx?id=76

Some vehicles already offer this feature. As for the concerns regarding the electric power required, isn't it electricity that powers most moveable side view mirrors?

powering a little geared-stepper motor in the mirror could account for something like three watts. Also, the mirror moves, but the motor is stationary. A heater of any size in a window won't do any good unless is something like 100 watts. At 12 volts, that means the amperage something like 8 amps and wire like 14 gauge. Attaching wires that thick to a window that has to go up and down thousands of times in the lifetime of the car...just a problem.

The wires wouldn't have to move. The set up could have contacts that only contact the window in the up position, like a roller at either end of the bottom of the window.