Car park does have an effect i seem to remember.
L5 75k fans, 21k stadium, ticket price does not matter cannot get more than 19.1k
What I have found is that if I reduce the ticket price from my current level $9 merchandise spend goes up whilst attendance goes down.
L3 is the only time i had full capacity, I added the full seating to the existing stadium, after one week I did the stadium upgrade then again and again, never sold out since.
I plan to do a full seating upgrade in a few days and I will run with it next season and see how it affects the attendance.