1992 Chevy C2500 Charging Problem

I have a 1992 Chevy 2500 2wd pickup truck 5.7L 350 TBI V8. I had problems with my charging system and replaced the alternator with a remanufactured alternator and it still has not resolved the problem. What the truck is doing is when its cold it will read good on the volt meter right at 14 in the middle maybe a hair past. Once I get going after warming it up driving in the dark turn the lights on heater on high (winter time so cold outside) the radio (aftermarket radio, rewired it 3 times) will cut off and turn back on when I step on the brakes trigger the brake lights and the volt meter is reading a hair above the red zone on the left side of the volt meter like there is not enough juice to run it, power windows roll up and down slower then normal. Also the dash lights headlights and heater fan drop in sync when the turn signals are on. Also replaced the battery. On my way to work I run into Tim Hortons and grab a coffee so I shut the engine off along with all accessories and it cranks over slower then normal. When I park it for the day while at work and go to start it to go home it spins over normal. Once in awhile the volt meter does a dance between 8 and 14 volts. The alternator has two wires on it. The red one of course bolted on the back of the alternator. The Brown wire, I am not sure what this is for and what its used for. I read its a switch to kick the alternator into charge mode? If I unplug the brown wire the volt meter reads the battery is dead when the engine is running but the battery light is not on. The battery light only comes on when you first turn the key on and start the engine. Once engine running the light goes out. Also the light never comes on even with the low readings on the volt meter. Grounds are all good I checked and cleaned them. I have a feeling this little brown wire is the culprit. Is there anything I can do with this wire, say ground it or run it to the red wire bolted onto the alt? Any input would be much appreciated.