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Hello I will help,
It sounds like you have enough current to run the testlight but not enough to run a light bulb. Do you have a voltmeter?
Put one lead of the meter to a good clean part of the body for ground and with the bulb installed and the brake pedal depressed measure at the green wire to see what the available voltage is. It should be close to battery voltage. If it is low you have a poor connection between the battery and the bulb so we would need to narrow it down from there. If you have battery voltage to the bulb you have a poor ground and then you check the battery to the body and the bulb to the body.
I think you will find you have a poor ground.
Let me know if that is clear and what questions you have, it is important to have the bulb in the circuit so it loads it. A straight pin can be used to get inside connections to measure but your light sockets are hard wired so you may have to pierce the insulation. When you get done measuring use some RTV to seal the insulation again.