Hey, take a look at my video.
This is what I made for my twizy, I chose to power it from an external lithium battery, It is very true what Osbrook says but it can be made to work with some ingenuity. I use mine mainly to heat the footwell as my feet were getting seriously cold.
You need to insulate the footwell area for this to work as there’s a lot of cold air that comes in from the front of the car, seal the hole behind the pedals is a good start.
I’ve been using it all through the cold period and it keeps my feet from freezing, combined with a heated seat pad running from the accessory jack (which only pulls 1A, very efficient) I can wear a normal jacket and be perfectly warm... oh yeah I also have twy rain windows
Yesterday when parked up charging it was 4 degrees outside and with the heater on I could no longer see my breath in the car. My heater is running around 400W on a 20V battery at 20A.
Hope this helps.