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It's likely that you are using some kind of relay (we use a trailer adapter) that merges the signal and brake light outputs to the single filament. It's also possible that the relay shows a voltage with no load but will not have enough current to light a filament.
So...
Remove the bulb from the socket, ground the outside and connect the brake light wire directly to either of the bulb's bottom contacts.
Does the bulb light on both the contacts? If so, there's probably a problem with the socket. If the bulb doesn't light, your relay may be bad or you lack good continuity in the entire brake switch-to-light circuit.
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