^^^^^^
So exactly like the 24v 80 then...... did Toyota not learn from the 80 ? There was a recall on 80's to change the relay .....
No. The heater grid relay doesn't normally fail. I have not heard of a single failure of this relay. But, in theory, all relays can fail, either by burnt out coil, burnt contacts, or just mechanically worn out. This might result in either a complete malfunction, bad connection (high contact resistance), or fused contacts where you get continuous power.
Ah ok, not wanting to sound stupid but is that the power heater that is switched from the dash?
No. This is an automatic, environmental feature, designed to reduce emissions by increasing the combustion temperature for the first 90 seconds of running, if the coolant temperature is under 60 degrees C. That means it will heat the inlet air even if the car is parked over night in 50 degrees C heat.
It is not normally needed for starting the engine. A direct ignition engine like the 1HD (as opposed to the 1HZ), with temperature controlled/compensated injection, does not need any glow or heat added at temperatures normal where people live. When the temperature gets really low, the biggest problem is that the diesel fuel starts solidifying. Toyota diesels for cold climates have a heated fuel filter to avoid that problem (down to a certain temp, and depending on the actual fuel mix for cold climes).
The Power Heater, OTOH, is well described in other threads.