If the speakers in the rear pillars are working than the signal from the amplifier (separate or integrated) is OK, they are both on the same output.
Things to check would be door wiring, especially the part that flexes when the door opens and the speakers themselves. You can get an idea of the health of drive units using a multimeter set to resistance, you will need to disconnect the speaker wiring to check it, otherwise you're measuring the resistance of the amp output and the pillar speaker. Healthy speaker should have a resistance of around 4 ohms and you may also hear a crackling sound from the speaker as the test voltage from the meter is applied particularly if you move the meter test probe around on the speaker terminal.
One of my rear speakers stopped working and I found that the solder joint on the speaker terminal was a dry joint, re-soldered and it's fine.
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