I agree with you that a 450kW truck engine would produce about 1MW of waste heat, but don't overlook the fact that the radiator only handles a modest part of that. The biggest heat loss is probably via the Exhaust gases
Ah, yes. Of course. My mistake. I do not know what fraction is removed by the radiator. I saw a paper on that some time ago . . .
This source says a car produces 42 BTU per horsepower, which isn't much: