When I drove a bobbed tail delivery truck some years ago they had brake locks on them. When stopped for a delivery a simple switch on the dash activated a solenoid in the brake line to lock the brakes, via the brake fluid, in the braking position. The harder the pedal was depressed when activating the switch the more set they were.
I have always thought I'd put such a device on my vehicle if in a theft prone area.
I have also thought about using air horns on door activated switches. Nothing more than a dome light switch would be needed to activate it, but you would have to route through a key switch on the outside of the vehicle to enable/disable it before entering. Air horns vs. common theft alarms should garner attention.
Perhaps there would be a way to keep the air horns blaring once the door was opened rather than have the thief simply shut the door to stop them.
With my flatbed, perhaps tie flashing clearance lamps into the circuit to also draw attention, perhaps a back up light also.
Perhaps tie in a circuit to activate spray nozzles via a pressurized tank containing a dye for marking the perp for identification. Expect vandalisim later!
If you can always park on wet ground (unlikely) you could tie in a flasher to an old coil with a positive wire to the soil. When the perp stands in the positively charge moisture and touches the vehicle he gets a coil fed jolt. We've all been jolted and know that it is no big deal, however a lot of folk don't like shocks. Famers use this to keep cattle away from their feeding vehicle.
The power lead from switch to starter on switch activated starters could be fed through 3 or 4 switches that have to be in the correct position to feed through all to allow flow. The switches could be labeled for lights and the thief might not ever realize they go to the starter instead.
Vandalism---that's an entirely different chapter
that has few preventative answers.
