I'd be very interested in what they actually find. These sort of problems can be elusive, compounded by varying levels of electrical systems comprehension and experience. Those electrons DO travel at the speed of light
Maybe they can give you a bit of a break on labor cost.
I'm the primary electrical guy at our Dealerships service department. I've got 40+ years in the field, and like a challenge (like it sounds like yours has become). Please post or email what they find if you would.