Lojack or GPS would be good but high dollar I would imagine (never priced one). I think a skilled high end auto thief would be able to bypass/defeat alot of simple alarms. My thought would be to add a common 1/4 turn ball plumbing shutoff valve in the fuel line, with the valve/handle under the seat but hidden somewhat & shut it off then assuming they bypass any alarm then several blocks away it will die in the middle of the street & no amount of cranking will get it going & they will likely panic & have to vacate. it might take several feet of added line to let them get out on a more public road & possibly a vent so all the gas in the line can be pulled forward by the fuel pump (I ain't sure on that as this is just a planned in my head addition), first I have to build something worth stealing! but JC whitless in Chicago used to offer a kit with a button you add on the steering wheel so if you were carjacked you could unobtrusively press the button as you exited the vehicle at gunpoint & it would electrically trip this shutoff valve to make it stall a few minutes later.