I am about 95% sure that parking is NOT good for Scary Farm. It's considered a special event, just like your usual AP/Membership does not get you into Scary Farm. It's a specific exclusion.
However, if you do park during regular daytime park hours and just leave your car there, that's fine.
Sometimes we arrive towards the end of the regular park day, have dinner in the area (at 30 Acres in the hotel or across the street at Claim Jumper), and then head into Scary Farm. Heads up if doing 30 Acres: on Scary Farm nights, we found out the hard way it gets very busy, and can have long waits, so arrive with time to spare. I seem to recall it took us nearly 2 hours to get seated, eat, and leave.