A speaker that does not work correctly may not mean that the speaker itself is the problem. Within the audio system of a car there could be other components that are causing the speaker to malfunction. Every car has a head unit that controls the CD player, radio and volume. However, some car audio systems also have an amplifier as well as a crossover. Take your time and test each component to make sure they are all working properly before you blame the speaker for the problem.
Turn the car and stereo off. Disconnect the left and right speakers at the amplifier. Connect the left speaker to where the right one was and the right to the left. Turn on the stereo system. If the speaker still does not, work then the problem is in between the amplifier and speaker itself.
Put the left and right speakers back to where they were in the amplifier. Make sure the car and stereo are off. Switch the position of the RCA cables in the amplifier. Put the left one where the right one is and the right one where the left one is. Turn the stereo on. If the speaker still does not work, the amplifier may be broken.
Test the crossover. The crossover will be connected to the amplifier and the head unit. Plug the RCA cables that are going into the amplifier into the extra outputs on the crossover. Turn on the stereo. If the speaker works, then you know it was the output on the crossover that was faulty.
Pull the head unit from the dashboard. This can be done by hand if the head unit is just sitting in place. You may need to have the head unit professionally removed if it is connected within the dashboard. Remove the RCA cables from the back of the head unit. Do not unplug any other wires. Put the RCA cable for the speaker that is not working in the output for another speaker. Turn on the stereo. If the speaker works, then the problem is either the RCA cable or the output on the head unit. If the speakers are still not working, then they may need to be replaced.
Work backwards by starting with the amplifier, crossover and head unit as opposed to starting with the speakers themselves.
Do not remove the door panel that is covering a nonworking speaker until you have eliminated the other causes for the problem, as the panel will be hard to remove and replace.