Probably the cigarette lighter if it's only hot key-on, easy to locate. Next seems like the radio feed, not the clock lead, that's hot full time. You can't find an empty slot in the fuse panel to tap into? You have a probe tester, one end clips to ground the other probes wires?
Some newer choke instructions tell you not to use the alternator field for power, but I'm not sure why. A good key-on power source on GM cars is the wiper motor, but I'm not sure if that's easy to reach on a Mustang.