I had been chasing down similar for about a year, but I'll tell you what it was for me.

There are 3 crimp connectors in the main charge harness, off the charge wire. One was corroded, and this caused every imaginable charging problem, from no charge/brake light with no charge to charge/brake light while charging, to overcharging with lights shining brightly (this ended up burning through the harness at the alternator connector).

Pull out the multimeter, and check for resistance from the main 80 amp fuse to the alternator. Mine would fluctuate some. I also had a .4 voltage loss from the alternator to the battery, and a 2.6 volt loss to the chassis power supply (the pink and green fusible links). you need to unbolt the fuse panel and check the voltage underneath. If you have any losses, pull the battery, and peel away the tape on the thick harness running around the battery across to the other side of the engine compartment (in front of the radiator). Look for corrosion on the 3 crimped splices near the battery.

For pics and more info, check out this thread from last week. There are pics of the harness, and what I did to correct the problem. I am now kind of the resident expert on internally regulated alternator charging problems. I have gone through the entire circuit, poring over wiring diagrams as I went along. It took me almost 2 years to chase this one down, so I hope this saves you some time. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Let me know what you find out... <img src="/forums/images/graemlins/cheers.gif" alt="" />