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Please play around with balance and try changing source Radio/CD. Also pay attention to tweeters to see if high frequency sounds on the left are working.
Once you observe no change, you may have to open up the driver door panel and verify the speaker is still good. Look for dry rot and check ohmage.
Next would be to check for break in wire ... specially around the door hinge as that gets flexed with every door operation.
If you get past those, the next would be the AMP or the headunit. The headhunt provides pre-amp out and you have to inject audio level signal to see if the amp is working.
If the amp works then it is the head unit.
Keep in mind first gen came with Pioneer or Nakamichi. So when you look for replacement, make sure you get the right kind as they are not interchangeable.