I have fitted one of these...
Realtime Auto Vehicle Car Spy GPS Tracker Alarm System Remote Control TK103B VG2 | eBay
I also added the optional shock sensor..
NEW!!Shock sensor shake sensor for GPS tracker GPS103A,B,TK103A,TK103B,best qual | eBay
I mounted them both in a waterproof case hidden up under the dash.
I use a GiffGaff sim card as the credit does not expire if not used.
My boat is kept on its trailer at my Mums house 80 miles away. I have it set on a power saving mode which minimises battery drain. Its been running for 6 weeks now and it still reports the battery is at 100% (but ive not tried starting the engine yet!)
The shock sensor is set so when the trailer is bumped its wakes up the unit and texts me its location every 2 mins.
You can also phone the unit and hang up, and its texts you its location in the form of a google map link.
I also have it set up on GPS-Trace Orange - free GPS tracking service | Free GPS vehicle tracking system, GPS trackers, GPS controllers
which is a free live tracking service using GPRS data connection. They also do a nifty free android app to track it on your phone or tablet.
I'm pleased as punch with the unit. So much so i have bought 3 more which are fitted in our delivery vans at work.
It comes with a hoofing great wiring loom, but you only need connect the two 12v feed wires and cut off all the others.
The handy remote on this "B" model means you can arm and disarm it without sending it a text message.
Let me know if you need a giffgaff sim card as i have some which gives you £5 free credit which you can have for nowt.