Gauge lights, Fuel and tach questions.
You may want to start with a schematic for your car.
Go to
http://www.mymopar.com/downloads/1971/71DartA.JPG and
http://www.mymopar.com/downloads/1971/71DartB.JPG
Check your firewall and ammeter connections first then check the fuseblock. I believe you're looking for power on the tan wire (at the fuses) coming from the headlight switch.
Your tach wire goes to the neg side of the coil.
Don't hook up wires directly to the ammeter unless they are fused.
Good luck.