See this page:
Although it is entitled "How to replace Lexus LS400/SC400 alternator, it also has some good tips on how to test the charging system as well as including a simple circuit diagram.
Note in the diagram that there are two fuses shown in the regulator circuit. You may have a bad one.
FWIW, that page explains how to R and R the alternator from under the car. I was able to change the one in my 95 SC400 quite easily from the top and in less time. It was much easier and cleaner as well as faster. (The pulley that provides the tension for the serpentine belt must be removed in order to lift the alternator upwards and out of the engine compartment. One bolt holds the pulley on, but be aware that it is backwards threaded.)
Here are the steps:
Remove belt by relieving tension (14mm socket)
Remove tensioner pulley (IMPORTANT--backwards thread) (14mm socket)
Remove alternator top nut (from stud) and then lower retaining bolt (14mm socket)
Move bracket that holds transmission cooling lines out of the way
Slide alternator forward off of stud
Turn it to get access to wiring, removing both connectors. (13mm box wrench and small flat bladed screwdriver)
Lift alternator towards center of car and up, turning as necessary.
Reverse process to reinstall.
It can be done in well under an hour.
(FWIW, I'm tall and bending over and lifting at the same time [I'm also old!] was difficult so I jacked up the right side a bit to make the connections easier to reach, but I did not try to access anything from under the car)