[mtdaniel71]

This is my first go round with a Chevrolet product. Had several Pontiacs & Fords. But always liked the looks of the 66&67. At any rate. The car is a pro street car. It had an electric water pump. I am converting to a belt driven pump. I have to switch the Alt. from driver side to Passenger side. The wire coming from the junction box to alt was too short. So I am adding length. That wire already had a splice a few inches away from firewall connection so I cut it there. Now when I slid the loom back after I cut it. Another cut wire fell out . Now I'm not sure which one I need to connect to. My choices are( Looking from engine bay to connector on firewall)
A. Bottom right Brown color
B.Second up on Right Black
That wire ends up going to the 2 wire Plastic connector that snaps on to side of alternator. It is the smaller wire on right side of attached pic.

 

[SouthBay2s]

Someone has done an internally regulated (SI) alternator conversion on the car using a home made method. Attached is a pic of what the stock connections to your alternator looked like originally. The large red power or "BATT" wire is normally left on its terminal and not moved to the plug-in connector. This is how many (all?) SI conversion kits on the market come and how I was taught to do it years ago. I am told what has been done on your car works, but can cause charging issues at idle. It is a "poor man's" one-wire conversion.

Onto your wire colors, the brown wire in the bulk head goes to the external voltage regulated and would not be needed in your setup. The black wire originally went to front lamps. Guessing it is not even original in your situation and has been added. The original blue and white wires tied into the external regulator, so that black wire is likely tied into the remnants of one of those wires somewhere in the harness. To answer your question, looks like you need to reconnect the black wire.

If it were my car I would replace the wiring. American Autowire makes a passenger side mounted SI type alternator light harness for your car #NV67819PI. It is a stock harness modified to work with your setup. Best of luck