I was once shocked when I was unaware of the private car regulation on Mt Fuji. There is a private car regulation for Mt Fuji Skyline and Mt Fuji Subaru Line during busy period.
Once the regulation is applied, all vehicles except permitted vehicle are prohibited to use the highway. The only transportation method is by the Shuttle Tozan bus, which cost about 1700 yen for a round-trip.
The regulation varies from each of the four tracks, thus better check before you start your journey.
One word can describe the Parking area of Mt Fuji 5th station when it is in its busy period-awesome.
It is not that packed in weekdays, but if you are talking about holidays, you can see cars that are unable to go into the parking area stop along the road for some few kilometers, which means you already have walked for a few km before you even reach the starting point. That's what I have seen during my trip starting from Kawaguchikoko.
To say at first, the only way is to park at the night before.
Normally, people who want to see the sunrise start their hiking from the night and leave the Mountain before noon the day after. This means that the number of vehicles will only skyrocket. After sunrise, this time is the normal climbers packing up the area.
That's why it is a tough job to find a parking spot in the morning if you are those who only want a day-return trip. As it is easy to predict the rush hours, the best way is to arrive at 5th station much earlier than that, and take a nap.