In my state the answer would be no, you can't keep a weapon in your car if the company owns, leases, or otherwise operates the lot/garage if the employer says you can't. State law regarding firearms doesn't override private regulations on private property.
I've confirmed this in my state with legal advice. Your state may differ. A cheap lawyer will answer this simple question for you for $100
Or you can go with the law of averages. If your company is large and employs lawyers of its own you can probably be assured that they've already determined whether they have control of the parking lot. So just ask HR and believe what they say. They could be conning you, but not likely.
I've confirmed this in my state with legal advice. Your state may differ. A cheap lawyer will answer this simple question for you for $100
Or you can go with the law of averages. If your company is large and employs lawyers of its own you can probably be assured that they've already determined whether they have control of the parking lot. So just ask HR and believe what they say. They could be conning you, but not likely.