Most likely cause is either failed wiring, a loose connector or a failed speaker in the door. The driver door is the most frequently opened and closed and each time you open and close it you flex the copper wire going to the speaker. Copper, while flexible, eventually can crack and break. I used to maintain scanning systems for papermills. The scanners move back and forth across the paper during the manufacturing process about once per minute. Even with the very best copper cabling - usually Gore, one of the many wires would break down and we would have to replace the entire cable bundle - usually around once each year. The inside door covers on these cars are actually pretty easy to remove and get inside - it takes about 10 minutes or less. An easy way to test if the speaker is good is to use a battery with a wire on the positive and negative contacts and to tap the speaker single input points. It should make a little pop without a scratching sound if the speaker is good.