The 12V needed for the FITech, is that going to be for the main power for the unit or as a on input? The reason I ask is the harness was never really meant to power all that much. The wiring is under sized and there's going to be significant voltage drop. What I did on my 66 was to split the connection where the pink wire plugs into the black wire on the ignition switch. The second wire I added controls a 30 amp relay that switches a fused 30 amp line off the battery to a auxiliary fuse block. I now have a good power source for anything that needs to have 12V with the key in the on position.
You don't want to tap off the coil even if there wasn't a resistor wire for a couple of reasons. The primary side has a high voltage spark or spike being produced there as well as the secondary side feeding the spark plugs. Not good for electronic equipment. The other is that the size of the wire is too small to feed both. You'd have a voltage drop problem. Another tip, if you install a relay put a diode in backwards on the relay coil terminals. The + of the diode to the - of the coil. The reason is the relay coil acts just like an ignition coil. Putting the diode in backwards kills the spike.
