HA! Here's one for everybody

[kylewhunt1]

70 Barracuda, turn the headlights on and the dang dome light illuminates. To top that all off, when you turn on the ignition, the fuel and temp peg out. Now how you goin' to solve that one?

[Bullitt-]

first impression on the fuel & temp is that the instrument regulator is stuck sending a full 12V to the instruments...see some details of how they work here http://www.allpar.com/history/mopar/electrical2.html
eliminate this issue with a modern electronic version  http://rt-eng.com/mediawiki/index.php/Image:ELimiter.jpg
only other explanation is that both sending unit wires are shorted to ground.

There is no reason the dome light should be associated with the headlights....someones crossed some wires.....get out the multimeter & start tracing the dome light..It should have +12V all the time & the ground is made by the door jam switches or the panel dimmer. 

  here's your diagram......have fun
http://www.mymopar.com/downloads/1970/70BarracudaB.JPG

[FY1Cuda]

  maybe a good place to start is to take the two thick wires (red and black) off of the ammeter and just attach them together with a small nut and bolt and some electrical tape.  The ammeter gauge has little spacers between the terminals and the dash cluster that sometimes fall out allowing a short circuit.  I was guessing that the thick red wire is somehow passing power to the fuel and temp gauges.  Bypassing the ammeter won't do any harm and sometimes even allows things to work better (by eliminating that resistance).  Keep us posted.  This is a good one.

[Cooter]

Well, the first thing I'd wanna know is how did all this begin?? Has someone been performing the "Weekend Warrior" wiring on this thing? been in a wreck? Piss your wife/girlfriend off?
I expect once we get the full story on this one, it might not be such a brain stumper afterall...

Sounds like you got a ground problem....The Domelight comes on as the headlights seem to be searching for ground and finding it through the dome light bulb as it evidently is grounded...

[MEK-Dangerfield]

Do you have a Gran Coupe, or a normal Cuda that I can actually understand?   

[dodj]

I would start with making sure the light switch panel is properly grounded. For the gauges, is it an original 5V limiter?

[kylewhunt1]

Ok, to answer some of the questions first off.
Pissed off girlfriend was good by the way.
For the serious parts, its a plain 70 Barracuda with the original wiring and the Rallye cluster. When I bought the car over 4 yrs ago everything worked including the back-up lights, which amazed me. Over time and about a year of hard storage little things stopped working. Some of the anomalies: battery would die if it set for longer than a day or so, it would only turn over when it wanted to on some days (this really confused me because I would get irritated and leave it for a few minutes then come back out and it turned right over), the gauges as mentioned earlier and the instrument back lights stopped working . Now the headlight situation with the dome. Obviously its a bad ground somewhere but its hard to find it. I bought the service manual, which is about the same size as the flight manual for a C-17 by the way, but the diagrams are generic at best.
Their were some fuses blown which I replaced but it seemed to make things worse. With the ignition off, once I attach the ground cable to the battery, I can hear a clicking noise that seems to be coming from the passenger side firewall. I havent found that one yet. SO....thats where I stand for now. I will try the mentioned techniques above and let you all know. I am in the middle of converting the brakes over to disc but if the wiring is faulty, its going to sit static! Sounds like a Painless Wiring weekend to me.

[Bullitt-]

  1st of all disconnect those gauges....if you keep sending too much power through them they will burn out.
   I really don't know what the problem really is but with all these issues I'd suspect the wiring may be melted & shorting in the main harness.... the bulkhead connector, right behind the fuse panel, has the charging circuit running through it. If that shorts & melts the insulation will create all kinds of problems. 
      Yea Painless.  .... 

[73EStroker]

Sounds like you have a lot of loose connections coming on. Bad grounds (as already suggested) and these loose connections will hurt you. Start at one end of the car and start checking.  One trick is where wires use the spade terminal into the female connector then squish the female terminal sides a little to "tighten" the connection up. Use some WD40 or even grease under ground connections and make sure they are firm. Don't try and tighten the hell out of them otherwise you get stripped threads. Also look for hot connections which show up as a blue or black color on the connections. In these cases you have to replace the connection or bypass. Loose terminals on an ammeter is a biggie. When I did my rebuild I undid the wires off the meter and firmly tightened the nuts holding the terminals into it. Then reinstalled the terminals and tightened the second set of nuts. So far after my rebuild and 2 years I have had zero electrical problems, but electrical is the most neglected or misunderstood system in older cars. Hopefully all this helps.