Sounds like a short on the positive side, I'd start looking for a spot on the main battery cable where it's touched up against the exhaust or rubbed through to the frame or body. The + cable likely got hot and melted because there was so much current going through it to meet the demand, even if that demand is a short, it won't know, it's just trying to do it's job. The tach problem may or may not be related, sounds more like a bad ground. As soon as you turn on the lights, the backlight in the tach should light up. If there's a bad ground it's going to seek a path, like the tach circuit board, now that will stop the needle from swinging. Could be I'm way off, but this is where I'd start, start with the current draw first. Sniper