Bruce,

The dash and door speakers are wired in parallel so if you're getting sound from the left dash speaker, the fader isn't the problem. Nor, for that matter is the left amp.

My guess is that something's not right with the left amp and maybe you only perceive good sound from the left dash speaker.

The amps are behind the seat back and cover the holes that allow access to the upper shock mounts.

Sixto
91 300SE
87 300SDL
83 300SD