The late series 2 car has the ballast resistor between the top of the fuse box (19) and the coil. I am just assuming, but I doubt it's behind the dash. Saying that I have never actually found one
There is another feed from 19 to a joint which feeds the warning light, but the light should only be on if both of the switches are on (the one on the choke pull, and the heat activated one on the manifold). Also, without the ignition on, there shouldn't be any power anyway.
Try this - turn everything off, close the doors and disconnect the boot light. There should be no power flowing now, at all. Removed the positive battery terminal and tap it gently onto the positive battery post (Where you have removed it from). If you get sparking, you have a short somewhere.
As Colin says, best to check for anything wrong behind the dash - disconnect the battery first
P6s often have electrical problems caused by previous owners, mine was so bad I called him Sparky.