I have a Marchal alternator on my /6. Coming from B+ on the alternator, there are two red wires. One is quite thick- this one obviously goes to the starter. Where does the other red wire from B+ go?

I believe that should go to the voltage regulator.
Here is a wiring diagram I got somewhere.
Tom

Is your alt an internal or external voltage regulated ?
This is what I have for info.

I have a external voltage regulated alternator.

It looks like the other red B+ wire goes to the rear fuse panel on a 911. I don't beleive we have that so should I just remove the wire?

I recently modified a conversion harness for another member. He had purchased a conversion harness for CSI. Since he was not going CSI, we needed to modify the harness for using carbs and a different dizzy. Had to remove some wires and add others. What I do remember about his harness before he installed it with the engine shroud was the alternator external wiring had 3 wires. His had a brown, green, and I am almost sure, blue. If you look at the 914 external regulator, it has 3 wires: brown, green, red. ( The red is hooked to blue at the relay board.) I expect that other smaller red goes to your external regulator. I remember his conversion harness had 2 blue wires going to the D+ terminal. One for the regulator and one to go to the 14 pin conversion connector that goes to the dash alt. bulb.
Please double check this before hooking yours up. It seems they are a pain to get to to rewire after the shroud is installed.
Tom