Sorry to read of the problems you've experienced. I'd be speaking with the employer though, as this can be regarded as a OSH/OHS issue. Essentially your employer does have a responsibility to provide a safe and secure environment. You might like to check with any community advisory service to gain an appreciation of both where you and your employers responsibilities lay in this regard. You've shown you have taken reasonable steps to mitigate the theft opportunity etc.; sought better parking arrangements - though this is an issue if outsiders can access what sounds like what should be a secure area; and bought (though not yet installed) an anti-theft device. You may find that your employer has certain liabilities that you're not aware of.
Good luck.