Diesel heaters are effective, but have you considered a 12V electric heater instead? It won’t be as powerful, but it could offer supplemental heat without the hassle of venting or adding fuel. Something like a PTC ceramic heater is safer and might work well for defrosting. Sure, it’ll still draw some power, but it’s less invasive and easier to set up compared to a diesel unit. Just a thought if you’re exploring options.
I haven't personally tried the 150W heaters but it would have to be very close to me and quite well placed to have a direct warming benefit. On the other hand, having a small tank of diesel to heat with is like having a second spark battery just for heating.
If I had to try 12V electric heating I would probably choose radiant heating over PTC convection heaters. With radiant heating, just imagine powering a heat lamp. Recently there's been a push to develop more efficient longwave IR heaters. Most of the infrared energy passes through the air to heat only surfaces in the path of its beam, so if you design it well, you can have instant heat, and very efficient use of power. The car will still remain ice cold everywhere else, though.
I should also mention that any heat not part of the HVAC loop will be heating moist air, which will be drawn around to the coldest surfaces where the air wants to naturally circulate. When the air drops across the windows, it'll lose its heat and then dump moisture that came along for the ride because the temperature dropped below dew point.
The climate controls mitigates unwanted dehumidification by running the A/C compressor to keep the cabin heat exchanger very cold as a moisture dump. The process needs to run together with the cabin blower fan so most of the moist air is fed into the HVAC loop rather than circulate over the windows on their own. You won't see it but water will bead up and accumulate all over the heat exchanger before dripping down to a drip pan, then slide into a drain that carries water outside underneath the car.
Smokey beat me to a response just now but he's right, it's tricky because windows are so good at being condensers. I'm pretty sure the Spark EV's glass is single pane too, so it gets an extra kick of winter power. Free dehumidification exactly where you didn't want it...