I am preparing for the cold days, so here is my setup:
1. upgrading the driver seat heating: I added heating panel also to the backrest, and connected both heaters to a thermostat for better heat comfort (not on/off every 5 min. like the factory setting.
2. I also added seat heating to the passenger seat ( seat+backrest), also with thermostat control.
3. heated steering wheel: it works fine in our nissan leaf, and it is doable in the miev. I ordered the heating pad, will start the project as soon as it arrives.
4. legroom: I installed 12v infrared heating film (220w/nm) under the front carpets - with insulation. It turns on together with the seat heating and controlled with a thermostat.
5. upper body: I also installed 12v infrared heating film under the front door panels - they also turn on with the seat heating, also has a thermostat control.
Total energy consumption: 25A.
The bigger project: I bought a bmw cabin pre-heater (it works in the BMW X3 diesels and in BMW Minis and preheats the coolant till the engine warms up so the cabin heating starts immediately). It works from 12v but takes about 40A - I did not measure it yet. Originally it is CAN controlled, but it is quite easy to connect the heating elements to 12v directly. To make it more effective, I bought the heating valve from a Ford fiesta - it opens/closes the cabin radiator according to the engine temperature. It is practically a 12v solenoid valve which closes down two pipes and enables the flow of coolant to another two pipes when powered. I plan to use it to shorten the coolant line in case I use the BMW cabin heater, so the coolant does not travel to the car`s main heater - however, when the battery needs to be warmed, the factory heater gets priority together with the factory coolant lines. This way I aim to somewhat temperate the cabin during the drive. I may use it for some kind of pre-heating but for that I need to upgrade to a larger 12v battery (the BMWx3 has a 150Ah battery, but the BMW Mini only has 55Ah, if I am not mistaken). Currently there is a 40Ah battery in my little car...
All above projects are are quite cheap as I am using salvage parts or new parts mostly from Aliexpress... the BMW pre-heater (used) was less than 10 USD, including shippment... unbelievable.