OK, the e_winter sleep is over.
I am trying a new DC to DC converter from MEAN WELL.
It is the HLG-600H-24A. 24 volts & 25 amps. IP65
I have seen it in Nick Smiths videos from Auckland, New Zealand. He never spoke a lot about it but I was looking for something better. Here you can find his videos:
First I connected my heated wire windshield (6 amps). It is 24 volt, this is why I keep the rest on 24.
Works pretty fine.
Now I run the whole 24 volt car system on the new converter.
When all the consumers are running (Including the hazard warning light.) I'm exactly on 24A.