Being a conversion and you say it only happens with the fan i would check the amp draw on the fan itself. Most fans are grounded by the relay via the pcm or ecu. It's possible you have a voltage drop on the positive side causing excessive amperage. Also check the relay I have seen funky things with relays that get moisture in them. It's kinda hard to diagnose without actually being there to see it. See if it starts when the fan comes on then quickly disconnect it before shutting off ? Good luck it shouldn't be too hard being it's narrowed down to the fan.
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