You are probably the only one who would know how your car is wired, being that it is a conversion.
If you have a modified wiring diagram, I would try to find out where and how that bulb might interact with the vacuum shutoff of the diesel. If the ignition switch was not replaced, does it control a solenoid for the vacuum shutoff? You might need to isolate the bulb with a diode. I suppose there are other possibilities.
Conversions can get pretty complicated.
Good luck with it,