First of all, company policy documents are often confidential. If you release them to an advocacy group or other entity outside the company you may be fired for that even though they couldn't have fired you for having a gun in the car.
The company can search your car if they want. They can't legally force you to let them do it, but they can certainly fire you if you refuse. The law doesn't say that they can't search your vehicle, it just says that if they find a gun they can't fire you for it. They could always say they're searching for illegal drugs (most companies have a drug policy), for lost company property, etc.
TX is a right to work/at will employment state. Employers can pretty much terminate employees at any time for any reason unless there is an employer/employee contract that says otherwise. So if you make a point of violating the policy and get caught, they may not be able to fire you up front, but they can certainly fire you later for something else or maybe just put your name on a list for the next time they do a force reduction or layoff.
The law makes it less likely that employers will try to actively enforce policies against guns in the parking lot, but it can't actually keep them from firing you. It might offer you some recourse if you are terminated, but it would be totally on your shoulders to prove, in court, why you were fired. That might be impossible and it would certainly be expensive and difficult.
I'd say that you should be smart about what you do. If your company actually has a history of searching vehicles and has publicly made a point of the fact that they oppose the law, then maybe it would be wiser to not leave a gun in the car than to take your chances. If your company never searches cars then maybe some folks in that situation would feel comfortable with having a gun in the car even if the company doesn't seem to be going along with the letter & spirit of the law.
Ultimately everyone's going to have to do their own risk assessment based on their personal risk threshold, their personal circumstances, their employer's past actions and on how the employer seems to be reacting to the law.