It is a good idea to knot the cord around something to take the strain if you
forget to unplug before driving off.
I have used a heater made for defrosting refrigerators or for scraping paint
resting on a brick on the garage floor directly under the oilpan, where I
just park over it. It is controlled by a switch in the kitchen, so I don't
have to unplug it. Admittedly a potential fire hazard from leaking fuel or
oil, but the garage floor is dirt, and it hasn't burned the place down yet.
I also keep a blanket of 3 inch fiberglass wall insulation, foil-side down
over the engine (diesel) fender to fender under the hood all the time in the
winter to help keep the engine's own heat in while running and afterwards. I
use a grille cover, and cardboard in front of the radiator also. I believe
that warming the engine prior to starting, and then driving gently til it is
fully warm, prolongs the life of the cylinder head, which is very much prone
to cracking in Peugeot diesels and turbogas engines, due I believe to the
stress of the steep thermal gradient between the exhaust valve seat the the
nearest waterjacket. That gradient is especially steep when the water is
still cold and the valve is cherry red under heavy load.
Brian Holm