Good afternoon.
The first thing I wanted to mention is that it seems you are discussing different things.
Sergio1671, from what I'm reading, seems to be referring to the ignition glow plugs as spark plugs. I get this impression because you also mention that your car model is a 1.9 D, meaning that being a diesel, it doesn't have spark plugs due to the compression and combustion system of diesel engines, which, as you correctly stated, only operate when the engine is started. And sosa10 is explaining the problem as being related to the ignition glow plugs.
Sergio1671, what are you referring to?
If it's running on 3 cylinders like you said, it's almost certainly due to a faulty diesel injector. What type of injectors does it have? It would also be a good idea to check if they're receiving power.
By the way, how did you do the test with the lamp? Because it seems to me that all you checked was that the heater was working properly and making a good ground connection. It also seems very strange to me that the air intake is so low; it should be behind the grille since that's where it draws in air best, if I'm not mistaken, next to the left headlight. Anyway, when you have a chance, check these things and clarify a bit for me too, as I'm still left with the questions I mentioned before.
I hope I can help you in the future.
All the best