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These instruments are grounded to the back of the instrument panel. It might be a simple grounding issue. If the gauge is a plastic type the mount is the ground. If its the older metal type the gauge itself is the ground.

Just about everything you mention is covered in the forums so use the search function to help with questions. The TM's (technical manuals) are also available for download on the site so take advantage of those. They will also answer many questions and increase your familiarity with the truck...