A lot of money has been wasted over the years from someone troubleshooting using the factory gauges. If you are worried about your truck's health and well being, get a set of aftermarket gauges and use them.
Not sure what year truck you have, but in 1986 they did have a underhood courtesy lamp that was fed by a lightgreen/yellow wire with a black wire. The lightgreen/yellow feeds all the courtesy lights in the truck at the door jamb switches, the glovebox, and map lights if you have those options.
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