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If you know you're having a problem with the ignition switch I would focus on fixing that first before messing too much with the glow plug system. There are may different circuits the ignition switch activates and it is theoretically possible that the switch could be affecting glow plug operation.
Also as far as the glow plug light goes how long are you expecting it to come on for when you turn the key to the run position to let them warm up before starting?
Just don't hit the accept button until the problem is fixed and you can come back here to this thread. One last thing though can you tell if it's the mechanical portion or electric portion of the ignition switch? I'm unclear if it's the electrical portion of the switch failing or if the mechanical cylinder isn't correctly activating the electrical portion.
Ok, in that case I'd maybe try to lube the cylinder a little or something. Another thing is the glow plugs are controlled by the engine computer so if the light isn't staying on after you start the car (indicating a fault) then their probably isn't one and you're just noticing normal operation of the vehicle.
Ok, well you didn't mention the rough smokey start. What do you mean by coolant glow plug harness. Are you just talking about the harness that goes to the glow plug? Are you getting power to the glow plug harness. Also have you checked faults in the engine computer?
Ok, I'm getting a better picture now of the situation. So, what you're saying about unplugging the coolant temp sensor is correct it just assumes that you're open circuit. Where did you get the temp sensor and do any faults remain if you erase faults and then drive the car around for a little bit. I know you stated you aren't getting power to the harness for the coolant but what about the normal glowplug harness.
You need to erase faults when you make repairs or the computer can continue to rely on old values sometimes. I'm not sure that would make a difference in this case. Is your sensor an OEM sensor? It is possible to get bad parts even new when they're discount parts. You don't have the ability to read temperature with your scan tool do you? You also could have a problem in the connector for the temperature sensor wiring or connector have you checked that? Erase faults and see if the fault returns.