A the battery
B the earth return on the alternator and the body
C the volt drop on the positive between alt and battery positive.
D take a reading on the warning lamp wire at the alternator if its above battery voltage you have a problem with another after market alternator not working correctly but also check the voltage/supply that is to the bulb.
earth return should be less then .2v
and volt drop on the supply should be less then .4v
just had a thought you could scope the positive and check to make sure the rectifier is working as ac volts could be a problem