First decide what the goal of the system is, it's the same for a house security system you first need to know and then decide what you want as the end result.
If you just want to prevent someone from driving it away, then you can install a relay that needs to be reset each time vehicle is turned back on.
This can be for fuel or spark or both, when key is turned off relay opens and without resetting it closed there is no spark or fuel next time it needs to be started, no forgetting to "set it" all automatic.
Motion detector alarms, horns and lights, can prevent break-ins or at least try to get a thief to leave before anything can be taken out of the truck.
Parking in a secure location, locked garage or hiring a guard to sit in the truck is about the best thing