The chief problem/consideration with portable GPS trackers is the battery life. It varies greatly among units - but is generally much longer with units that have motion detectors. If you're looking to track only one vehicle, then your best bet is a hardwired vehicle tracker. But you will probably need a professional to install that. In any event, you will need a "service plan" which will probably run you $25-$40 per month.

Alternatively, there are dozens of applications that can be installed on a person's cell phone - without the person knowing. The only caveat here is that you need to know the person's password(s). Just Google it and you'll have a gazillon different programs to choose from. My personal favorite is the one from Highster Mobile, cellspyexposed.com for $69.99 (all in, no monthly fee). If the person has an iPhone and is not particularly tech savvy, you can use the simple (and free) app Find my iPhone.

We have find my Iphone on all of our iPhones

Find my iPhone is a good app, but is extremely limited. All you can do is, well, find the phone. If he's banging his secretary at the office, Find my iPhone won't do you much good.

Other apps are far more robust, and allow you to access emails, text messages, photos, etc. Some go so far as to permit you to remotely access the phone and turn on the camera and microphone, so you can watch and listen in real time.

you have to get the phone away from them to "jailbreak" the phone to use those apps