No big deal but i have a quick bulb Q?

On my wife's 2007 Santa Fe LTD the inner right rear stop list seems to be out. But ONLY when the headlights are not on (i.e.: daylight).
Someone reported that her light was out and she had me check it later
that evening. It worked. But then she turned off the headlights and it no longer illuminated.
I'm not famililar with the bulbs (usually they are dual filament - one for tail light, another for stop light). But this appears to be stop light only.
Before I go galloping off to the dealer does anyone have any experience with this situation and therefore have anything to enlighten me with?
Thanks for all who ca reply :(
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I'm not an expert, but I've seen some strange things like that with automotive lights. Replace the bulb and see what happens. Check the other bulbs too as they may be in series. The dual filament seem to be more prone as filaments can break and fall across the internal wires of the lamp.
How big a deal is it to access the bulbs? Some can be a real PITA to get to.
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bobmct wrote:

Its either as Ed says or the assembly lost its ground and feeds into the entire light system when the tail lights are not turned on. You can easily check it with a volt / ohm meter.
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