I use Carrotech trackers on my cars, no commercial connection apart from being a satisfied customer. You can set them up to e-mail or SMS you if the ignition is switched on, or more significantly, if the vehicle is moved without it being switched on. Positional accuracy is five to ten metres anywhere which is great, and they are small enough to hide inside a chassis rail, although you wouldn't want a lot of metal between them and the sky, so some sort of sealed chamber inside the chassis rail at the end of the vehicle would be ideal. You can fit a GPS ariel but then you have to hide that, and the lead, too.
I do remember they would work down to 6 volt, but check that, and the unit cost was reasonable, plus a yearly sum for use of the SIM chip and tracking system - overall not too bad.
The one disadvantage is that they did drain down the vehicle batteries. I got mine set to check in with base at a lower frequency which cut down the drain, but it was still significant. I'd think on a jeep you'd need to have them wired to the vehicle, of course, but something like a solar charger in place to provide a constant small charge to make up for the tracker drain. I don't think a separate battery would work unless you were really sneaky about it - and it was huge.
Ask for Russell and tell him Gordon sent you :cool2: