Improper car speaker wiring can cause a number of problems, including:

1. Poor sound quality: If the speakers are not wired correctly, the sound quality may be distorted or impaired.
2. Amplifier damage: If the speakers are wired to the amplifier incorrectly, it could cause damage to the amplifier.
3. Speaker damage: Incorrect wiring can also cause damage to the speakers themselves.
4. Electrical issues: Improper wiring can create short circuits, which can cause electrical issues and potentially damage the vehicle's electrical system.
5. Fire hazard: In extreme cases, improper wiring can create a fire hazard.

It's important to make sure that car speakers are wired correctly to avoid these problems. If you're not comfortable wiring speakers yourself, it's best to have the job done by a professional.