It was quite interesting to find a good place for interior electric heater. But here is the solution. The backseat armrest. Look at the pictures.
I tried to write this in German language, but it is more than 30 years since I had it at school. Sorry for that. Aber ich habe Deutch gelernt.
The cup-holder at the backseat has just perfect sizing for usually user home electricity wall bases here in EU. I think that Siemens and all the other producers have these at stock at you nearest market. All you need to do is to make a plate with some extensions to put the air blowing upwards. Look at the pictures.
We had her in Finland minus 30 decrees this winter and with this help my wife was able to go to work every morning with open windows. So there was no need for me to wake up and clean the windows for her.
Best regards: Mokka_EFJY