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disclaimer: not an electrician or an expert but this is what I think I know. mods please edit if information is incorrect
so the battery has a positive side and a negative side. each one connects to either pole of whatever you want to power. the negative side is often wired to the frame - instead of running a positive wire and a negative wire to whatever you need, you just run one and connect the other side to a ground, it makes it loads easier.
if something is touching the frame and grounding itself at a point before it reaches the bulb, it'll go the path of least resistance instead of lighting up the bulb.
so you can have a perfectly good bulb, perfectly good relay, but a stray wire can cause problems.
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