Low voltage to the exciter wire on starter

I've got a 4 cyl 2018 Dbt Cab SR, and for the last year and a half or so, I have been having starting issues. When I turn the key, nothing happens, similar to a bad starter. If I push on the brake and/or gas pedal a few times, it will turn over. Took it to a mechanic friend who looked through, and couldn't find anything. He diagnosed the starter as being OK, but told me the vehicle was getting low voltage to the exciter wire on starter, usually after a drive cycle. The truck always starts after a few attempts, but I'd like to get to the bottom. I recently purchased the vehicle from a family member, who babied the truck, but he does live on the Mississippi coast, and there appears to be more rust under the hood than a 2018 should have; nothing terrible, but noticeable.

Wondering if anyone else has experienced anything similar? My uneducated brain thinks it could be a wire somewhere on a rusty ground, but I really don't want to take it to the dealer and say, I've got a wiring issue, please diagnose and fix - way too open ended and ripe for a multi-thousands of dollar repair.

 

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