Is it legal for your employer to force you to get a tracking device installed on your personal vehicle?

Can you answer craig79792000’s question about GPS?:

I do a lot of field work, not going to specify the profession. There is a lack of supervision because I am always out on my own. I get my job done, bottom line. Does it seem strange that an employer might require a gps tracking device? I think they are passing it off as a safety issue, but what about weekends and time off? I don’t want my personal business to be available to my employer. Any thoughts?

Golf Caddie Gps