A couple of things. I got interrupted in reading this thread but I think you said you had a multi meter. Even a $3 one from harbor freight will work. Put it in amp mode and see how much current you are drawing during the different scenarios.
Brut force testing. The wiring will handle 10amps even if the load is suppose to only be 5 amps.
Now back 50 years ago when the yellow cab radios weighed 50 pounds and were the size of a small suitcase, I was at the radio shop and the worker was trying to track down why the 50 amp radio fuse was blowing. The owner told the guy just keep putting in bigger fuses til you can follow the smoke. You gotta do what you gotta do.
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