Alternator not charging

I have a charging problem it may be just a wiring problem but not sure right now, maybe someone else has had the same problem. I installed a chrome Powermaster alternator on my truck on the alternator there are two wires the battery feed and a brown wire I checked the voltage at the alternator and its 12.0volts not running and the same when it is running. I've checked all grounds to the frame,body,battery everything checks out ok my question is could the brown wire be causing the problem or maybe do I have to ground the alternator cause it has a chrome case around it?

Year/Model?

Sorry I guess that would help it's a 91 GMC Sierra with a 5.7litre.

where does the brown wire go to?