With the black ground wire plugged in? Was the lightswitch on?
Perhaps it would help if you were able to visualize what's happening with this basic circuit:
When the light switch is turned on, electricity flows down the red wire from the chassis (lower right in the diagram), into the 4-way connector where it splits off to the right and left bulbs.
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Once it reaches the bulb, the electricity flows through the tail lamp filament and then to the outer metal shell of the bulb.
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From the metal shell, it flows out through the black ground wire to the aluminum rear tub, then back to the battery using the metal of the body and chassis as a "wire".
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As you can see, the brake light does the same thing on a second filament inside the bulb, but uses the same outer metal shell as its ground. If that ground is sufficient to complete the electrical path for the brake lights, it is sufficient for the tail lights as well.
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Presuming that the black wire was connected and the light switch was on for your test, the only thing that could stop the electricity from illuminating the tail lamp is a poor contact between the lamp socket and the bulb, as described by hillstrubl, or your bulb is burned out.