Dude, I am principled as the next guy - or even more... but if you owned the company and provided the phone, service and covered the cost of that AND is paying you for your time... it's hard to argue that if you're the "man" ... you have a right to know where the employee is - right?
What I've found is that most GPS tracking efforts are aimed at getting a product delivered, or a customer serviced or managing excessive costs for overtime and fuel. To do this better, the owner usually needs to know where his people or trucks are - right?
Unfortunately, it's not unusual to find employees taking two-hour lunches, stopping by strip clubs, heading to a girlfriend's house for a pit stop... or any variety of things. Sometimes it's just a need to know what's going on while he pays the bill.
If all of this sounds ridiculous - I'd quit and search for a better job with all kinds of increased money! But be careful... if the new company asks why you left and checks on a reference, you might just have to face the reality of the new company who wants to improve on their services / response times and save money too.
Another thing... the next company might have the GPS unit in the truck without telling you. At least you're aware of the current process.