I have a 65 mustang with the trditional Falcon speedometer and idiot lights. My alternator light is coming on dim as it may be). The battery seems strong but headlights look to be a little dim. What are the possible reasons and cures?

This weekend the the radio went from loud, to quiet to out. The turn signals dont work and the lights are dim. I pretty much believe it is the Alternator or the Voltage Regulator. Am I on the right track?

 

Possible causes:
1)loose belt
2)bad alternator(or going alt)
3)bad voltage regulator
4)low battery voltage.
How many volts does it show at the battery while the car is running?

 

Thanks for your help, it was the voltage regulator