Those stereos also have high output alternators and capacitors to help the charging system do its job correctly.
I had no idea they drew so little. I just know from personal experience, when it is cold outside and the heaters are cycling, my headlights dim and my FASS drops PSI. I wouldn’t really want to add more to it then necessary. Although, if the water pump doesn’t start spinning until a certain temp is reached (like some V8 gasser electric water pumps I have seen) then the water pump might start just about the same time that the grid heaters stop cycling, either from vehicle speed or temp.
Heck, I may even have electrical problems that I don’t even know about yet, and that is why I see so much draw.