Since his truck is an 86 it would have an internal voltage regulator. The only trucks to my limited knowledge that had external were 79-81 for the 4x4's. I had this same problem and it was the alternator quality, like the others said, get a genuine unit, in fact. Better off getting genuine toyota everything for your truck, you really get what you pay for when it comes to parts. That stereo must be dimming your headlights! If so i would run a capacitor, or larger battery if possible, or else you will keep killing the low quality alts with that much current pull. Even the stock would have a hard time with its small output (guessing around 45 amps for the runner)  I installed I cs-144 gm alternator from a buick roadmaster. I had to swap the pulley which is quite simple. This may not be an option for you but I yanked my dead a/c compressor and used the bracket to mount the new gm unit.Rumor has it you can fit a big case gm alt in the stock location with some ninja-tuity, but I would recommend you upgrade your stock wiring to handle the x2 increase in current to the battery. If all of this sounds too intimidating to you, you can do like I did and have an electrical shop do the conversion, I had an external regulator and I am colorblind so I had issues          eiether way it can be done and I highly recommend it. My output is now 145 amps at idle, much more than necessary for my needs, but I also have a 11000 lb winch going in soon.  Hope this helps!