Hmmm, the harness, such as PN8874 allows you a quick and dirty hookup to the ford tfi ignition if this is for a 5.0. This allows you to remove it quickly w/o issue in the future. It is not the "cleanest" harness connection I have ever used but I did go the quick and dirty route using it cuz I can remove it and go back to the oem set up easily if I think I am having a problem, or want to hook the car up to a scope w/o out the 6al hooked up (3 patterns per plug gets confusing).
The 6al can be installed w/o harness. You have a heavy black and red which go to battery ground and positive; the other smaller wires such as the orange wire from 6al goes to (+) on coil, the black to (-) on coil. The small red goes to the connector removed from the coil via a jumper to the red&lite green side (+). the white goes via a jumper to the dark green&yellow side (-) of connector. There is another wire coming from the opposite end of the 6al box that is not ussually used. I'm no expert, all my installations have been on pre 1990 Ford trucks and Mustangs, but I am looking at the tech tip book from MSD and do not see any reference to a switched 12V source (although the coil connector is switched)I hope this helps.