I got 10 meters of the cable. I had some red cable from Biltema before that was 6 m in length, but it turns out it wasn't quite long enough. I hope 10 will do. Maybe I should have measured how much I need. I also got some clear 40 mm diameter tube to use as a conduit from the back of the car to the front. I got the idea from this Damien Maguire's youtube video:
Very nice, clean and hopefully durable solution. My cabling from first go from the trunk to below the read seats, where there are a couple of holes already available for bringing the cable down to the underside of the car and then using the exhaust pipe tunnel into the front.
The heater in the picture is a cheap 1000 W cabin heater. I'll see if the heating element would happen to work on 80 volts DC as well. The fan most likely will not work in any case.