What’s to torn about? Quoting you:

a parking lot is an employer’s property. But my car is my property

That seems like a very nice bright-line divide, doesn’t it? Inside your car, your business. Get it out, and it’s your employer’s business. Why is this hard or complicated?

Can anyone tell me how common it is for employers to search their employees’ cars for guns? I really can’t imagine it is common at all. I agree that the inside of my car is my private property, regardless of where it is parked. I still maintain that this law is a solution looking for a problem. Its one thing to follow the letter of the law… we should all do that at any expense. But I could give a damn about following the letter of a stupid company policy when the odds of being caught are nearly zero.

This assumes that you are able to go elsewhere. For many people it is impossible or impractical, for one reason or another. People just entering the job market may have only one or two offers, and bills are piling up. People near retirement age often have difficulty changing employers. Many areas only have one employer in a given field, and changing employers would require paying moving expenses and uprooting the entire family.

There can be many reasons one’s choice of employer is limited or nonexistent. In those cases, the employer CAN force you to be disarmed. And that force extends far beyond when you are actually working, as Harold pointed out.

Employer can search my company email, their server and computer. They can search my desk at the office. Their desk. They cannot search my pockets, my car. I have a right to have anything in my car that is normally legal. Books or guns.
If a drive a company car they can mandate what I have in it or not.

My personal civil rights do not disappear as a result of being employed.

They can fire me if it is an at will employment. This law simply clarifies that an employee cannot be fired for keeping legal property in his car.

They are a lot of laws that prevent arbitrary dismissals. This just adds to those laws.

Can an employer fire you for lawfully protesting in front of their office?

I was pretty sure they could…

If my employer tells me they don’t want me to have a gun in my car, and insist on searching it (which they can because they are covered under one of the many exemptions) then I will simply take my skills elsewhere.

On the other hand, employers requiring vehicular searches for their employees are rather rare. Disney, who in part sparked this legislation, only searched for and found the now-ex-employee’s gun after he told them it was there. There never has been wide-spread abuse.

This law was more of a solution asking for a problem. And it is a poorly executed solution at that.

If we think it’s good public policy to allow citizens to carry concealed, why would it be good to prevent that from happening for much or most of their weekday?

I think this pretty easily tips over the question of “who’s liberty interests governs?”