Re: 84 K10 Lighting problem, this is weird
^ I agree ... this sure sounds like a ground issue to me as well.
There should be 2 ground screws on the core support... one on each side. Start at the headlight connector and follow the wires back if you're having trouble locating one of them.
Since the ground wire on each side is shared by the headlight, parking light, and turn signal, there is still a connection between those lights even if the connection to ground (via the core support) is lost. That allows the head/park light on that side to find an alternative ground path back through the front turn signal circuit, causing the signal indicator on the dash to glow. Removing the bulb from the parking / turn signal socket breaks that alternative ground path.
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