Taillights sometimes work, then they both go out, and come back

Hello, does anyone know what is the most likely intermittent connection in this taillights circuit? Is it 1) oxidation of the fuse block (spring connectors at each fuse end), or something within the headlight switch? - It is always BOTH taillights (the dim filament of the double-bulb). Brake lights always work, turn signals always work, headlights always work - the problem is only the taillights. Thank you for any ideas! Rich

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