Wiring Problems

I have a 88 b2200 and have been having electrical problems. The truck starts and runs just fine but when you cut the headlights on (which are predominantly stuck on bright for some reason) they start out bright and finally dim out and then just cut off. I have recently replaced the alternator and battery. I also replaced the headlamp switch assembly. Im out of ideas can any of you guys point me in some direction. All help would be appretiated

if u got a new alternator from advance or auto zone somtimes u get a bad one. id have it tested if you know FOR SURE that all your connections including chasis ground are good n tight and have a good connection. id take it up there and have it tested if you think you can make it. does your charge light come one when its running with the headlights?