True - Most folks who live in northern climates (Michigan, New York, all of Canada, Norway & Sweden) who drive iMiEV's on a daily basis have installed tiny diesel burning water heaters in them. The iMiEV heating system (such as it is) uses a very ordinary heater core from the ICE version of the car with a 5 Kw heater to warm the water and then the fan blows air through the heater core just like in almost every ICE car. Using that 5 Kw heating element to heat the water severely reduces the cars overall range. If it's really cold out, you may lose 1/3rd or more of the cars roughly 65 mile range just heating the cabin
Fortunately, since it's using hot water to heat and defrost the car, you can very easily heat the water using other means. The little diesel heaters (smaller than a shoe box) will give you a very warm car for a week or so using about a quart of diesel fuel - These are a very popular mod for iMiEV drivers in cold climates
The air conditioning system on the other hand is very efficient. We lose 10% or less of the cars range staying comfortable on 95 degree South Mississippi days. In addition to our TC which very seldom gets driven, we have a 2017 Chevy Volt for longer trips, but about 85 to 90% of the miles we've driven over the past 7 years have been in our two iMiEV's. They are so good at what they do that there's not much need for any other vehicle, day to day . . . . unless you need to tow something or take a long trip out of town, which is why we're hanging onto the TC
Don