sorry for the delay. on holidays. You want to check and see if the glow plug wires are supplying power. First disconnect the wires from the fuel pump, no more fuel is needed now. Remove the wires from the glow plug, two push on connectors, put a meter on them and see if you have voltage. Should be near11.5 v. If you have power then the glow plug is dead, open circuit.If you have no voltage then the ECU is dead.Do you have two pumps or one? Most d12s have a primary pump and the secondary ESPAR pulse pump( the one the tics fast) If you have no voltage then the ECU is dead. How many times has the fuel pump pumped for the complete 3 min. start cycle? If it is more than 4-5 times, it could be dangerous to start it now. Espacially if the exhaust insulation has absorbed diesel from the exhaust joint at the elbow. The elbow should have a copper drain coil on it to drain off excess fuel in the elbow, it should be dripping diesel out the bottom by now. If you have started it many times it should be stripped down and have the excess diesel drained and cleaned out. If the exhaust insulation is wet, it should be washed with deagreaser and rinsed well before starting it up again.
The thermostat issue is probably not related, unless you have low voltage to the heater, or corrosion on the plug at the heater that supplies power to the thermostat, or a bad rotary control(h/l dial)- you can unplug the white plug at the furnace and jump neg(brown) to mauve- makes it run on high. and then connect red to blue, that makes the heater start- this bypasses the thermostat and rotary dial. BUT do not start it it it has many starts of fuel in it.
There is no D12l anymore so your only choice is a D8LC. 8 kw versus your 12. What kind and size of boat is it?