I've installed Frost Heaters in our VW diesels. The Frost Heater is just a installation kit built around a ZeroStart engine heater. In the instructions it says not to use the heater while the engine is running – I get why there would be little point in this – but what I'm wondering is whether there would be any harm in it.
The heater is thermostatically controlled so it is likely to shut off when the engine reaches temperature anyway (which seems equivalent to unplugging it). So I'm trying to figure out if I need to worry about it or if I can start the car and then unplug it, or even leave it running for a few minutes to help warm up the car if I didn't start the heater soon enough.