<VV> Battery drain source?

[Grant Young]

I received a call from an owner who winters in FL about whether the neutral safety switch on his 1963 PG car could be the cause of his battery draining. He recently installed a new battery. The car will not start in P, but will in N, and otherwise runs okay. If this could be it, what is involved in changing the switch. (Doing this long distance for him, so I don't know any more). Any suggestions are appreciated as he is not able to get under the car and I might wind up trying to help out.
Thanks,
Grant