The motor is bad only if there is a dead short, meaning 0 ohms, or an open, meaning infinite ohms.. A test light will not work to test a motor unless it has been disassembled.
The case of the motor is the return (ground) path to the car's body. The motor must have continuity, with low ohm readings, through both the field and armature windings to work. The loss of either is a failure. The motor must be disassembled so the windings can be tested for continuity individually. At this point a light will show continuity but not a dead short. You will need to employ a VOM (volt, ohm, meter) to complete the diagnosis.
The VOM set to ohms should show some but less than 10 ohms through each components windings. There is only one winding for the field magnets but there is a winding for each segment pair of the commutator on the armature. All must be checked for continuity and also for 0 ohms to the shaft of the motor. If there is a short from commutator segment to motor shaft that is a failure and likely not repairable unless armature is rewound.
The likely failure is the spring loaded brushes are not making good contact with the commutator. The brushes may be stuck in their holders, badly worn, or dirty along with a dirty commutator. Good luck.
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