Any plastic polish will do, but some are made specifically for auto plastic. Brands like Turtle Wax are a good choice for this.
If you’re using a towel, rather than a buffer, to rub the toothpaste in, apply toothpaste to a small area of your tail light at a time. If you put too much on the tail light and can’t rub it in right away, you’ll end up with dried-on toothpaste. Consider taping around the tail light so you don’t accidentally get toothpaste on the car’s paint.
