Wiring harness is new..... Probably go 1 wire ....
A three wire alt will actually perform better than a one wire, reason being the ďsenseĒ wire can be run to the main electrical junction under the hood. The alt will maintain ~14.2v at that junction, so there is less voltage loss throughout the rest of the system.
With a one wire alt the voltage regulator gets the voltage reading from within the alt and maintain ~14.2v output from the alt. Any additional load on the system, including voltage drop through the wiring up to the main junction block is not recognized, resulting in dim lights, weak ignition, etc.
Below is a pic of my junction block, mounted on the firewall. It runs at ~14.2v, no matter what. This means my lights are brighter(those would have been ok anyway, I run relays on my lights), but more importantly the fuse box (just a few inches of wire away) runs at full power all the time or very close to it. Dash lights, a/c fans, ignition, etc. all good.
I modified the circuit found on this website to work with my particular specs:
MadElectrical.com - Mad Enterprises
I called and talked to Mark for three hours straight, he is a wealth of info.
All that being said, I now have a bulletproof electrical system.