I use my lovingly restored optimus 45 pressure stove, to be replaced by an optimus 00 when i get round to restoring it !!
What about a Crusader mug and stove? You can't get anything much more simple and it is not expensive plus you can use chafing gel or hexi as fuel which is really handy to carry. Clearly if using hexi you need to be careful with the fumes but you are not likely to be running it in the car anyhow.
I have a few stoves but love my Crusader and over the year it gets quite a bit of use including for making tea at the back of the car.
I use my lovingly restored optimus 45 pressure stove, to be replaced by an optimus 00 when i get round to restoring it !!
that is on my short list. it will be either a crusader mug and stove or an Esbit 585ml Cookset Stove (very similar idea to the crusader) or I can use my gas camping stove and remember to be careful about storing the gas in the car ( I have a black car that can get very hot during the summer)
I did originally think a DC kettle may work well but the reviews seem to show they take an age to boil and there is always the danger of running the car battery flat
Back in the day ...