You need to install a relay. Relays carry heat loads better than switches. If I understand what you are saying you are wanting to run a hot wire to the glow plugs keepin them hot all the time. This could cause a catastrophic engine failure if a glow plug melted from constant heat and broke and fell in the engine.
Glow plugs do not need to be engerizered more than 8-10 seconds at a time, not constantly.
Glow plugs do not need to be engerizered more than 8-10 seconds at a time, not constantly.