The employer only has a say on what you have in your car if your car is parked on property they control. Your employee handbook can only govern things that you do while at work or conduct that affects the company (e.g. social media use). If your car is not parked on company property, they can go and pound sand. Now if the shopping center owner has a policy against it, you would have to find streetside parking on the days you want to take your tools with you.