Hey new member here guys at MONTE CARLO FORUM Have 72 Monte Carlo with battery drain over night, It has external voltage regulator ,3 wire alternator . When I check the blue wire on plug at alt with test light there is power with no engine run or key on would that be a drain. When I remove plug from alt no draw on battery.

 

I take it you do not have a service manual for your car?

 

I am gonna say that the blue wire should be switched through ignition which would stop the battery drain. Good luck.