A friend of my dad is starting to distribute a system which you install yourself - you simply find a hidden spot in the car and stick it there. Think theres a worry that the fitters can immobilise the system straight away once they know where it is! (one M3 was nicked from its owner in the UK a day about being in the garage - tracker turned off within minutes) Its being installed on new cars by Mercedes and BMW too so it must be quite decent.Might be one of the ones you're looking at.
Might be getting one of these myself in the near future though, so if you do go for one let me know how it works out! Mine has the Tracker in it at the moment, but thats no use unless I'm in the UK
Decided to go with the Clifford instead, the trakers look good however they can be disabled rather quickly, and if there is no signal, Ie they pull the antenna out you can not track the car. The system do work in ireland however, They use GSM rather that GPS.
The other down side is they can only be imobilised once the ignition is turned off, they can not restart, the Clifford will imobolise the car while its moving....