'91 gsx stock stereo (the one with the little EQ in it and seperate CD). The left dash and door speakers are barely audible, but occasionally after some crackling they come on for a period of time. I checked the speakers and the connection at speakers and head unit, all ok. All other speakers ok. Could it be an outage in the head unit?

After ten years of listening,your factory head unit is starting to give up.start shopping around for a new unit.