Theres cheap trackers available on ebay/amazon etc,
Only £25 or so, they're dead easy to fit, only around the size of a iphone so easy enough to hide,
They take a sim card, but the cheapo vodaphone sims from poundland work perfect, top up with £15 credit and they give something like 300 txts plus the £15 worth of txts,
They track to the exact space the car is parked in, very accurate and very easy to use,
I've fitted around 15 to a fleet of hire vans i look after
And they are all still working perfect a year on
They also have extra functions like reading fuel levels and listening devices to go in the cab but i've never botherd with any of that,
They do have a built in immobiliser that can be activated by txt so you can shut down the engine at any time
They realy do work perfect and weve never had a problem with the unit or the tracking
i'd use 1 of them before an old iphone