I recently replaced a plastic headlight motor gear with a brass one. On inspection it became obvious why they fail and it is not the design or manufacture of the gear that is the cause of the problem. The worm and wheel design should always have a bearing at the opposite end to the motor.
The problem is that the worm relies entirely on the motor bearings and the motor is only held in place by the steel motor cover that is crimped to the bracket. When under load the wheel pushes the worm away putting side load on the motor casing which works loose. when there is enough play in the motor the wheel will start to jump on the worm and it is this that causes the plastic to wear and fail.
A brass wheel is stronger at the outside of the tooth so stands more motor deflection.

If you do not crimp the loose motor body it will eventually slip on a brass gear or the motor could fail.

If your car headlamp motors work without making a horrible ratchet sound and you are still on the original plastic gears just check the motors for any side play. If you feel any play take the motor off hold it in a vice and gently punch the four metal tabs that hold the motor cover to the bracket.

The plastic gear and headlight motor should last years......unless you forget to lubricate the headlamp hinge.