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In the mean time to help somewhat, start by disconnecting anything external such as a remote control panel/switch and if the problem stops then check that switch panel and harness, if not then check the rest of the wiring for shorts. Since this is only happening with preheat you may want to isolate that circuit checking for shorts. It could be a problem with the actual circuit and not the wiring such as a glow plug.
I am just seeing if you still need assistance with this problem? The reason I ask is because I see that your question has not been answered sooner and I wanted to make sure if you still needed help.
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