Major Electrical Prob!
 

Ok, i am about tired of my car doing this. ever since i bought the car when i turn the headlights on either my dash lights go out or get really dim, sometimes they go out and my 2 blinker lights come on. But others it works just fine, i have narrowed it down to a bundle of wires that come in through a firewallfrom under the hood on drivers side..But not sure where or which one 's it is specifically to try and fix. I am so confused on this and just tired of it..And ideas besides something shorting out.Another thing is my tach is stuck when i turn the car on at about 4500 rpms, when i go over that it begins to move , but stays there when i am under 4500, but whats weird is when the car is turned off it drops down to a no idle position.,.,.,PLEASE HELP

Have you checked the grounds. Having a ground problem will cause all sorts of weird stuff to happen like you described. what kind of referance material do you have for wiring diagrams?
Another thing you may want to look at is your multifunction switch. Ford routes alot of circuits through that switch (parknig lights, head lights, turn signals, etc.)
Another thing mustangs are notorious for is bad harness bulkhead connectors and you may have an intermittent open/short in one of those wires especially since they tend to vibrate and rub together. The only way to isolate it is really to pinpoint it to a certain circuit and section of wire.
I've been looking through my Chilton's book (which sucks BAD!) and it looks like alot of the main light circuitry goes through that switch and since the manual doesn't show interior lighting i don't know if the multifunction switch is in any of that circuit (dash lighting), but it may carry an essential ground....i'll try and look up some more if i get an opportunity. Keep us posted!

Major Electrical
 

Thanks for the info.I am guessing i will dedicate this weekend to finding this problem..It seems like it is putting a major strain on my battery, due to the drop in gauge when the lights are turned on. Do you think this would effect my Tach also?