Bigzjn

Have problems with my 2000 Gs400 having problems from low voltage from the years just playing factory to aftermarket radio loud in traffic and battery drain quick making second thought of adding subs but after many years several batteries now i am having major drainage issues. First car shut off to complete drain, bought a battery drove on for a while to drain issues. Found out had low voltage issue so took out alternator to 2 different locations for check and turn out good and ran extra power source from alternator and added a extra ground. Next morning went to get charging tested and turned out fine until i went to the city and battery died again. Had the battery charged and turned out new battery went bad so replace. Drove home still no voltage, so added new alternator and following day got the car checked and still no voltage. So Baffled to the point to have some advice.