Speakers not very loud

Does that mean the other speakers work properly? There are multiple versions of the radio system. If your car does not have a remote amplifier, the best suspect is broken or frayed wires between the door hinges. Some of these radio models have a protection circuit built in for the radio's output circuitry. If a speaker wire is shorted to ground, the protection circuit will disconnect the radio from the two speakers, but the shorted wire will bypass part of the protection circuit, so there will still be some sound.

Normally you will find six volts on both speaker wires, then the symmetrical but mirror image audio signals appear on those two wires to provide full output. When the protection circuit kicks in, there is no audio signal on the shorted wire, and zero volts on both speaker wires. The full half of the audio signal can't be developed on the good wire, so volume is very low.

On most of those radios with the protection circuit, once the short is removed, you have to turn the radio off, then back on, to reset it. As I recall, the protection circuit only operates when the radio is first turned on. If no short is detected at that time, full power goes to every speaker. If an intermittent short occurs after that, it is still possible to cause damage to the radio.

In nine out of ten radio models, there is an output IC for all of the left speakers, and another one for all of the right speakers. There were a few models that used one IC for the front speakers and the second one for the rear speakers. The protection circuit deactivates one entire IC when a short is detected on any one speaker wire.

If your car does have a remote amplifier, it is possible it has a failed IC. It would be smart to go to the two weak speakers first, disconnect them, then measure the resistance to ground on each wire. Take the measurements while opening and closing the doors or when flexing the harnesses between the hinges. If no short to ground, and no short between the wires is found, suspect the amplifier.