i have a mustang haynes repair manual but the wiring diragram's just look like a big mess to me.

 

it looks like that to everyone dont worry. that box is your voltage regulator. key advice when you change starters change the solenoid when you replace the alternator go ahead and replace the voltage regulator. if none of the guages are working and there are no cuts in the wires go ahead and change that black box.

 

im having a similar problem with my guages in my 66 f/b ive checked all the fuses in that little black box and they are all good. nothing electrical inside my car works except for the lights in the back seat when the doors are open. should i replace that little black box? or is the problem deeper than that?? ALL my outside lights work btw

 

This is the same problem im having in my 65, no interior lights work at all, except the ALT light(properly working). Ill check my fuses, but im almost certain my problem is bigger than the fuses. I remember seeing a tiny bit of smoke come out of my dash area when all the lights stopped working -- my gauges work for the most part though. Lighter works, etc. But no darn interior lights.

 

The gauges have a seperate voltage regulator that's mounted on the back of the instrument panel. I'd start with that.