Sounds like a common feed that's missing. Given that the engine trim works, but the throttle one doesn't I'd be looking for a missing feed rather than a bad earth. As Willk says, you need a meter & a bloke who knows how to use it.
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Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: Don't feed the troll
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