Your gun is your property. I dont want it. I also may not want it at my place of business. If I make it clear that I dont want these on my property, you DONT do it. You may not like it, and to many it may not make sense, but you have not right to thumb your nose at your employer's rules because you want to bring your guns to work. And yes their are plenty of places that do a check of your car before you enter a workplace. If its a condition of going onto the property, then thats life.
You may not like not being able to dictate what my personally owned vehicle contains, but this is a personal problem you need to work on. If the contents of your employees conveyances is such a concern, buy them company cars to commute in. Just realize, that their safety too and from their residences is yours as well, since you won't allow them personal firearms for protection. So you may want to contract a private security firm to shadow them while they commute.
Or, you can just stop being an arsehole and let your employees exercise their Rights.
You can also list your business at anticcw.com.