I am in my second week of swapping a 289 from a 66 fairlane into my 67 mustang. All that is left is getting a short identified in the wiring ::. I have it narrowed down to two wires that come off the bigger post on the side of the starter relay that the battery cable attaches to. One is the yellow wire and black with yellow stripe wire that goes at some point to the regulator. The other one is a solid red wire that I can not find listed in my 67 schematic. The only red wire I can find listed in that location goes to the ammeter in a 66 schematic. My car was built in Nov of 66, so would it have still used a 66 model year wiring harness? I don't know if it will help me but I am grasping for anything at this point. Whenever I touch either of those wires to the post they are supposed to be attached to with the key turned on, the battery drains immediately and the regulator makes a clunking sound as sparks fly from the post. I have replaced the starter relay, had the starter and alternator checked, and replaced the alternator harness and starter wire. I have also traced the wires through the front of the radiator to the regulator looking for a crimp or open wire. If the red wire is to the ammeter for the 66 schematic, then I may pull the instrument panel and look for a problem there. Any electrical experts would be welcome to weigh in!