They can't detain you or force you to open the trunk--you can just drive away. But they can fire you (or suspend you, or otherwise discipline you) if you won't let them do this, unless you have an employment contract protecting your job. Without a contract, you are an "employee at will" and may be terminated at any time, for any reason, including that you won't let your employer search your vehicle on demand. So you can refuse to be searched, but risk your job in doing so.