12v auxilary coolant heater from a BMW

[oil pan 4]

You might not have to ditch the idea.
You could work your way up to it, find a bigger alt, put LED lights where you can (brake lights), install a volt meter, install a heavy duty or deep cycle battery next time.

This isn't one of those plug and play mods, a few other considerations have to be made.

I forgot to mention, that with my intake heaters (draw 80 to 100 amps), I burned the factory wires up where they plug into the alternator. Fixed that by using bigger wires and better connectors.
It had a AWG size 12 wire, from the factory which is less than 4mm^2, I think its more like 3.3mm^2, replaced that with AWG size 8, which is something like 6 square mm cross section.
No more problems with that.

[Cobb]

Im lost, how is having an electric coolant heater that runs off your battery help mpg? How about more eco mods like block the radiator, engine blanket, higher temp t stat?

[oil pan 4]

I think he just wants to get up to temp faster. For heat. Its cold and wet over there in the winter. I have spent quite a bit of time in England, but luckily none during winter because I have not herd good things about their winters.

[cRiPpLe_rOoStEr]

Some extra insulation in the engine bay would also help to get a faster warm-up.