On a Sienna 2003 both tail lights are not working is there a fuse that could cause this?
Yes, in the fuse panel. Open and read your owner's manual.
What could be the cause of your high beam lights working and the low beams not on your Chevy Malibu?
What would cause daytime running lights and check engine light and power windows and instrument gauges and lights to quit working as working?
What could cause the dim lights not to work. Brights are working and driving lights are working in a 94 grand prix Is there a dim light fuse?
If the motors are working, then there has to be a problem between the motors and the lights. If they are only "working" though if you bench test them, and they don't actually do anything when you turn on the lights, then there must be an electrical problem such as a break in a wire or a bad ground to the motors.