We had a long narrow bar element toaster, about 500w per side. One side failed, so I converted it to a storage heater by inserting a spare rosemary tile instead of a piece of bread and disabling the 'pop-up'. Strapped it to the front of the driver's seat and fed a wire back through the boot to a socket via a time clock. Result: warm car, clear screen, heated steering wheel and some stored heat to the backs of thighs until the car warmed up.

Neat

When I was sleeping in my Nissan Sunny while doing an MSc at Aberdeen University, I sometimes used a big pop bottle in a sock as a hot water bottle. (Yeh, I know. I'm a wimp. Even more so now).

Anyway, if I wanted to use the car the next day, say to go and get a greasy supermarket breakfast, I moved the bottle to the dash top while I went for a shower. The residual heat was enough to gently clear the windscreen inside and out.

Pop bottles won't take boiling water, so it was probably only about 70 degrees, but a lot of it, maybe 2L.

Not so high tech or ingenious as the repurposed toaster (Do you actually need the tile? Seems like it'd work faster without its heat sink?) but it doesn't require mains at the car, which of course I didn't have in Aberdeen University car park.