There is one and only one 12v wire in a classic Mustang that is hot only when the key is in the "on" and "start" positions. This is the red/green ignition wire before it goes into the pink resistor wire.
Every other wire in the car is eitther not 12v or is hot with the key in the accessories position or with key in off position (this includes the threaded stud on the back of the ig. switch ... it's hot in the access. position as is
every fuse in the fuse box.)
The problem with the "I" post is that is it's only about 6v ... won't heat the choke fast enough and will open late (won't have any effect on the ignition system including the coil, though).
The problem with hooking to the stat post of the alternator is this is unregulated voltage and will often exceed 12v, heating the choke too fast and making it open too soon.
If you always do what you've always done,
You'll always get what you've always got
http://www.classic-mustang.net/johnpro
http://www.classic-mustang.net/john66/mustang/pics/66fsides2.jpg