If an employer can tell you what you can have in your car then he can wreak havoc with your life, what if he doesn't like baby seats, how do you get your kids to and from school or daycare? Can she ban the bible? or leather seats?, can they ban tire gauges, how about spare tires?
If she owns the parking lot, she can ban anything she wants. If I don't like it, I'll have to figure something out. There are infinitely many options, but a few include: work for someone else more reasonable; comply with the rules; park nearby and walk; take a bus; carpool; some combination of the above.
That would be weird, but what would prevent an employer from imposing such a (dumb) rule?
Can she ban the bible?
Not on its own, but they could ban all books.
or leather seats?,
Sure, why not? There are employers that ban leather from their premises.
can they ban tire gauges, how about spare tires
Sure.