96 lexus ls400.Replaced alternator 2x, and battery, and neg batt clamp, alt fuse. Still getting voltage regulator failed message. Cause? Bad ground or short? Bad pigtail?

SOURCE: my 1991 lexus ls400

The most likely problem is your alternator, the alternator charges your battery while the engine is running, if the alternator is bad you have a dead battery, but you can check with a volt meter to ensure that is your problem. Start your car (use a battery charger or jump to start it if you need to) and put your meter on the battery post's. If your reading 13volts or so your alternator is working and maintaing voltage, however if your reading starts dropping to 10 or less(may take a few moments to drop) then it's probably your alternator. You can also check your battery's voltage before starting your vechile, if the voltage increases after you start the vechile then it seem to be charging. Alternators can be repalced somewhat easily, it's only two boltsm however you do have to remove the fan belt, and re-route after installing the new alternator. If you have some expirence working on cars you can do this yourself in your driveway.
Good luck

Here's a start for you, hope this helps

SOURCE: charging system

3 wires for the altenator all controll the voltage regulator. You need the ground as said and one is 5 volt constant and the other is 5 volt switched.

SOURCE: elec problem between alternator and battery

The Problem was that the plug-in that goes into the altenator was not getting a good conection. Drove the van after that I had it fixed and the battery light came back on and I checked the altenator and the plug-in had fallen completly off. Plugged it back in and made sure that it snapped in completly and now it is fine.

SOURCE: 2003 VW Jetta Charging problem

problem is that the voltage regulators on the 1.8t which I am sure is the same on the TDI alternator....is that they may work fine when they are cold....but once they warm up is when they start acting up and causing problems. I assume is has to do with some resistor over heating. This is just a common problem noticed by most people over at VWvortex.com.
So you can take your alternator to get tested but if it is tested cold then well it may appear to be working ok. When in reality it is broken. Trust me...it is a cheap enough part and easy enough to fix to not just try it. Some can even change the regulator without taking the alternator out. If you have small enough hands I can see it being done.

SOURCE: I have a lexus ls400

re please and grease the inside the wire harness be for putting in bulbs.