is it running/charging right now? & what is the charging voltage at a fast idle? I'm assuming electronic reg? either something dead shorted in the blue wire ign1 (run) circuit or it full fielded. I'd do a quick visual on the affected area for anything that ain't kosher/grounded/loose etc especially the alt brushes/brush holders. Clarify the location of the burnt areas & on the bulkhead do you mean the eng side or the underdash side. Also at midnite with everything off pull a batt cable & see if there's any spark. Did the amp gauge peg recently? Did you use the vehicle to charge a depleted batt.