[DeTomaso] Weird Charging System Problem

[John Buckman]

Sean,

Disconnect the connector at the voltage regulator.  Turn the key on (engine off).  Is the gen light in the dash on?
If so, then the wire (blue / black) between the gen light and the voltage regulator connector is grounded somewhere.
If the gen light is not lit, check for + 12 volts at the "A" terminal (yellow wire) of the voltage regulator.  
Perform this check with the key off and the 4-wire connector disconnected from the voltage regulator.
If this checks ok, then make sure that the wire (green / red) from the voltage regulator connector"s "S" terminal is 
connected to the alternator's "STA" terminal, and that the wire is not broken somewhere.
Next check the wire (orange) from the voltage regulator's "F" terminal is connected to the alternator's "F" terminal, and
that is not broken.

If all of this checks ok, then take your alternator and have it tested, and see if they can check the voltage regulator also.
From your description, it sounds like either an issue with the wire to the regulator's "I" terminal, or the voltage regulator.
You could possibly have two bad voltage regulators.  Ask for a voltage regulator for a '71 Mustang.

John