69 valiant electrical gremlin turn signal and license plate light

Depends what you attach the jumpers to.
The goal is to bridge the gap.
We don't know exactly where it is, just that its somewhere in the return.
You find it by sytematicly trying connecting different points in the ground path.
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Whatever you connected the test lamp to must be a good body ground.
So begin with connecting one end of the jumper to that.
Then connect or touch the other end to different parts of the lamp holder. See what happens.