Some numbers for comparison, these are not exact numbers precisely calculated or measured but should give general picture.
4.2l D2 engine consumes 1.4-2l of petrol in idle, average energy density for petrol is around 8.7-8.8kWh/l so if we take average fuel consumption in idle at 1.7l/h it produces almost 15kW of energy every hour at idle. Most of this energy is heat, some goes to drive gearbox and some is used as mechanical energy to turn the engine but converts back to heat from the friction. Part of the heat comes out through the exhaust but most goes in coolant and oil, i am going to guess at least 10kW. How long would it take for engine at idle to get coolant and oil to working temperature? - I have not tested but I guess quite a while. When you compare 125W heater it's only 1.25% of the heat that engine transfers to coolant at idle.
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The Webasto aux heater is a lot more powerful, wow 5kw I didn't know
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Auxiliary heaters are normally in range 2-5kW for passenger cars depending on vehicle and engine size. In case of Audi of this vintage (A6/A8) it's 4kW for diesel and 5kW for petrol.
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Remember an A8 engine is typically 200 to 300kW and this is just a couple of light bulbs
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It's 250 or 265kW for S8 engine in D2, but that is only at full power. Cruising at steady 60-70mph it will only produce around 20kW. Nevertheless internal combustion engines are only 30% efficient at best of times, typically you are looking at more like 20% in which case for every 20kW of mechanical power there is another 80kW of wasted heat produced. Part of this goes in the exhaust and part in cooling system(typically 1/3 of total power so around 33kW in this example) while small part is conducted to surrounding air. How much goes in cooling system and how much in the exhaust depends on operating conditions- at low rpm and low load like idle more will go in cooling system, at high rpm and high load(full throttle) more in exhaust.
At full power efficiency is a lot lower, maybe like 10% so engine producing 265kW of mechanical power will produce 2400kW of heat but at these conditions most of it will go out of exhaust and less in cooling system, still possibly 300-500kW or more.